I think a lot of the complaining comes from people with a long history on eBay. In the early days, they charged a small insertion fee, a 5% (or so) final fee, and otherwise stayed out of the way.
Then came PayPal, which was bought by eBay, who proceeded to all but ban any other form of payment - and charge you for two separate fees on the same transaction. The fees steadily went up. And a lot of regulation came along as well - buyer protection, seller protection, return policies, detailed star ratings, multi-tiered seller categories, etc. Not all bad things by any means, but a shift of control over the transaction from buyer-seller to buyer-EBAY-seller, and a lot more energy spent on compliance. About a decade ago, I sold something for about $2.00, got a check, deposited it, sent out the thing, and a month later got the check back from my bank with a $20 bounce fee. That wouldn’t happen today with PayPal. On the other hand, unscrupulous buyers and sellers have worked out their ways to game the current system (the discussion boards are a veritable Soap Opera Channel of treachery and despair).
It’s also very off-putting that every time they hike their fees, they send out an email saying that they’re lowering the fees (e.g. “We’re lowering insertion fees by 5 cents!!!” Final fees go up 75%). They “encouraged” offering free shipping by applying the final fee to the shipping charge - so if you sell something heavy for $50 but it costs another $50 to ship, you’re paying double the fee.
The market has also been swamped by large-scale sellers who flood the site with countless duplicate items - try searching for a whistle, and look at all the Susato knockoffs you have to wade through, even though they represent just a few distinct items. Musiciansfriend and Music123, owned by the same parent company, each offer the same items for the same price, and on it goes with all the big box/online retailers. All this makes it harder to find that unique item.
And then there’s the frustration that there’s no better alternative. Amazon charges more in fees, on average. Other sites like bonanzle don’t have nearly the traffic. So eBay uses its position to maximize profits, as all corporations do.
I offer all this as an explanation, based on my experience buying and selling for a dozen years or so (I don’t sell for profit, mostly just unload books/cd/movies/etc that I don’t want anymore, in order to fund the new ones). Given that I’m still an active user, I’m hardly anti-eBay. But I get the frustrations that have grown among people who’ve been around a long time. There are a lot of other gripes people have, some legit, others petty, but it all boils down to a lot of people feeling like they’re paying more for the privilege of working harder for a less enjoyable experience.