I've been selling on Tradesy for a couple of years. I liked it at first, but they kept upping their commission. At this time they take 19.8% from the seller. That's quite a chunk of change!
This meant I had to raise my prices to make a sale which resulted in my sales dropping. Drastically! The other problem which is deeply frustrating is that it takes about a month after a sale to receive your funds. I'm accustomed to emailing their staff regarding that, but I get the same canned response about it taking "7 to 10 business days". I wish!