I've used e-Bay a lot for both selling and much more for buying. I've had a total of 400 transactions and I've got 100% feedback.
They have the biggest customer base.
I've never had a problem either buying or selling. Most vendors will move heaven and earth to ensure their customers are happy, as they don't want negative feedback.
It's the first place I look for anything, as you can determine the lowest price available. eBay list stuff in price and date order. i.e. the lowest price will be at the top and as the sell date closes that will make it's way to the top. However, progressively, some items are "sponsored," (it says so in the listings), this means they will be at the top, but they are not necessarily the cheapest.
Sometimes the prices offered by eBay sellers, who may also have their own website, will be the same as stores in the high street. "The dealbreaker" for me, is if they offer "free postage." I always pay by "Paypal."
If I want to sell something, I look on eBay to see if I can find a similar item that someone else wants to sell.
I then go into "Completed Listings" on that page to see what price similar items sold at or didn't sell.
That will give me an idea what I'm likely to get for it.
If someone else is trying to sell the same item, I've two choices I can either offer mine at a lower price, or wait until that one has sold, before I put mine on.
eBay have been charging commission on postage for a decade.