I think the most common method of achieving that many sales is to get a buddy (or 2nd account) and sell stacks of 10 salvage back and forth -- at least that's the way I and a few friends did it back when the badge was new.
Used salvage because it's easier to manage 'stacks' of 10 at a time and get that much credit. It still takes some work, but it's doable.
As far as 7,000 sales in a normal play setting? I can't see it for most people. Strange that the devs didn't revisit the selling badge totals in the recent badge reevaluations.
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StainedGlassScarlet - L50 Spines/Inv Scrapper | Badges: 1,396
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Got it. When WW came and fabricator was 10000 it was converting, making things and selling it. At that point it was no need to buy/sell stacks with a friend as long before you got Fabricator you would have gotten Power Seller.
Why should they change Power Seller, if so they would have had to change more of them (and there are rewards for getting the sold badges vs most of the achivements (with exceptions)).
For each 1000 item you sell you get a badge and for 1000, 3000, 5000, 7000 you get one extra slot in WW/BM.
Its more of an incentive to get those slots I think 
But to get it as with most things... Enter the world of WW and buy and sell stuff.
Good luck 
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Yes. Think most of it was selling what I had made trying to get all the other invention badges.
Here's what I did for it (this only really works if you don't mind spending a lot of influence).
Go to a salvage type that has just about equal parts available and people that are buying. Buy up as much as you can for anywhere from 100 to 1000 per (or whatever your limit is). Then turn around and sell for 1. You should see your stacks get sold pretty quick. Buy some more until there is no more left or you reach your point of not wanting to pay for it. Rinse and repeat for that type of salvage. Move on to another type.
Sure, you'll spend a lot of money, but you'll a) get your badge; and b) help others get theirs.
Actually I got it both sides legit. Its not all that hard, just takes trips to the market to sell your drops. Granted I was also selling base components (buy base salvage, craft into components, then sell for profit). It does take time, but with the new team setting rules, you can fill up with salvage fast.
Net
Tanker Tuesday #72 Oct 5 @Champion
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Two of my main marketeering characters have it. If you flip salvage as part of your income, you're pretty much guaranteed to get it.
easy way to get power seller:
flip stacks of cheap high volume salvage.
bid 10 or 100, re-list for 20 or 200 (or whatever).
Fill up whatever slots you don't need with bids, start turning them over and it adds up fast.
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how many people have that? 7000 sales just seems like a lot to me, but then again i have rarely used WW's until recently when trying to get the invention badges on my 50 kin.