Copake has a 20% buyers premium. Don't know the seller's, but I'd guess it is similar. Adds up compared to a private cash deal.
People complain about ebay but their 10% plus the 5% a paypal is a pretty good deal.
Correct. If the price you acheive is 50% more than you would otherwise (which I think eBay often can be), then you are still ahead after paying their fees. Look at the "Barry Scott gambit" for example.
...and PayPal isn't 5%. As a heavy user, I pay 1.9% plus the 75 cents or whatever. So maybe 12% total in fees when selling on eBarf.