Doesnt StockX have something to the effect of a 13% fee? Hearing that drew me away from that platform and others like it. I may just put them up on ebay.
According to this link on StockX's site (click for link), fees start at 9.5% and there's also a 3.5% processing fee, so you'd be correct. Grailed has lower fees (click for link), and the platform's legit, but I haven't sold in a while there, but it wasn't a bad place to sell the last time I used the site. Had an overall decent experience.
Ebay's not that bad, but I find the site is sometimes lacking with seller support. I've had problems with that in the past, but I've been selling for a few months without issues on Ebay. Might be worth a shot, and I recall Ebay fees are lower than that of both StockX and Grailed.