Where are the casual players?

I'll point out the Goat's Rule #1: Anything good with zero available for sale, is priced too low.

To define "good", look at the list Bionic Flea gave out.

To define "Priced too low", look at the last 5 and price a smidge higher than that or if there is one much higher than the rest, aim there.

This rule is virtually never wrong. The very few times are where you're talking about low-level versions of something that is good, but not popular. Like a level 11 basislisk's gaze.

As you can see, there are a few different philosophies of the market and, though it's not obvious,a few different skill levels.

Here are my recommendations:

1) Sell all common [white] recipes to a vendor unless you're planning on crafting it for personal use, like a Plasmatic Taser or jetpack or something.
2) Check "# for sale/buying" for every yellow and orange recipe. If there aren't any buyers and there are a ton for sale, vendor it. If there are a lot more buyers than sellers, it's probably worth selling. I have a bunch of mental shortcuts like "don't bother with yellow mez recipes" but that's probably more complex than you want, starting out.
3) Usually, if it's worth selling it's worth crafting. Check to make sure.
4) The Penny Tray Rule: Figure out how much inf is "like a penny" to you- for me it's about 5000 inf, for some people it's 500 and for some people it's a million. You know those "Take a penny, leave a penny" trays? Don't worry about taking or leaving a few pennies of inf.
5) Everyone who actually sells lists under "last 5" prices and some people get complex trying to figure out how low they can list without actually selling at a lower price. "List low and sell fast" is generally good advice. If it didn't sell after a week? Relist it.

AE farms don't drop purples. Trials are a very poor place to get purples because the kill rate is so low. I like to do the hero tip missions or any level 49+ mission at +0/8. Purples have a drop rate of something on the order of 1/2500 or 1/3000 kills so just kill a lot. anything 49+ can drop them.

if you can get a team of at least 4 players and run hero tip mission at +0/8 and your team as a whole should see 1-3 purples a week... assuming you do 5 hero tips per night and try to kill everything you can. ITF and LGTF are nice for this too but again, try to kill all for the drops.

Thank you so much for all the advice! It is exactly what I'm looking for.

But one thing that still has me scratching my head is this: how does one know if a piece of salvage is "junk"? I mean, short of searching the market UI for each and every piece of salvage there is--which sounds rather tedious and time-consuming to me, but what do I know?

Looking at the benefits--and therefore the likely market value--of a crafted set IO is fairly discernable just from looking at the description text of the recipe. But the description text of any given piece of salvage tells you virtually nothing useful. It seems to me that there is no "quick estimate" heuristic for vendoring vs. selling salvage short of becoming a human database of salvage lore. I'm pretty certain I won't be crafting or marketeering seriously enough to acquire that lore on my own.

I kinda like gameboy's idea of just selling anything that isn't purple for 1 influence (yeah, I know he said everything, not just purples, but I'm hypothesizing a minor variant here). Isn't it mostly true that the amounts in question surrounding non-purple recipes aren't usually worth trying to min-max on the market, particularly if one is not a hardcore marketeer? It feels to me like purples are probably, by and large, worth my time to check out their true market value, compare crafted vs. raw recipe profit potential, and maybe even try to gauge supply and demand forces. But anything less valuable than that probably isn't worth that much effort in the greater scheme of things, right? (Again, speaking from a casual player's point of view).

I mean, I just turned around a single purple enhancement for nearly 720 million. Compared to sales like that, it is hard to get too worked up over a difference of a couple million here and there on the "small stuff", between dumping it on the market for 1 infl and trying to game the market for maximum profit. Please understand that I'm not trying to find the Perfect Marketeering Strategy; merely absorbing everyone's insights and advice to see what feels like it will blend most comfortably with my personal definition of "casual play".

I don't know, but off the top of my head, around 50% to 90% of the price of the crafted IO.

It might help to know how salvage is organized. [my brain is going "There really IS a lot of salvage, actually... doesn't seem like it."]
There are three tiers: low (10-25), medium (26-40) and high (41-50). There is slight overlap (20-25 enemies drop low or medium tier salvage, for instance.)
In each tier, you have six tech and six magic of each type. Circle of Thorns drops magic, Rikti drop tech and so forth (a lot of 'em drop mixed types.)

Bottom tier, almost everything is "penny rule"- people just don't craft a lot of level 20 IO's so there's very little demand. There are a few commons that sometimes sell for decent prices.
Middle tier, highly variable. Generally the rares sell for 1 to 2 million, but uncommons and commons can go for 10K or 200K, and it can change from morning to afternoon.
Upper tier, commons are "penny rule". Uncommons are variable. Rares sell for 1-2 million.

Be wary of TFs that advertise as 'speed'. There is a chance that the leader will check your info to see if you can keep up. I have seen players not even invited or kicked off because they didn't have enough set bonuses. 'Master of' runs may be especially picky too.
Before even doing the trials, unlock Alpha so any incarnate xp you earn will apply to Judgement, Lore, etc instead of the Alpha slot.
Expect to repeat trials to collect incarnate parts. The trials are fairly short so you can do 1 or 2 based on your time.
Like other people said, IOs and trials are optional. But if you want the bonuses and incarnate powers, you gotta put in some effort.

Um, what is a "Master of" run? And why is a Mind Dom so important to one?

Well all of this raises an interesting question (to me anyway). And that is, how do speed runners and Master-of team leaders know who is experienced enough to be able to keep up and who isn't prior to inviting them to the team? Presumably these teams prefer players who know how to handle themselves in these high-challenge scenarios. Since there is no easy way to advertise that you are an expert or that your build is all IOed out (or that the opposite is true), what is the usual recruiting method that maximizes success?

Do players respond with tells saying things like, "Hey I'm a Master-of expert, please invite me", or "I'm a Master-of noob who needs experience, would you take me"? And what sort of response do tells like the latter typically elicit?

My experience has been that alike people tend to gather together. So speed runners tend to know other speed runners. Once a core group forms and they know each other it has been my experience that it doesn't matter who the last few slots go to, unless they are attempting a master run.

The speeders I run with are happy to team with anyone that is willing to try.