This may be more of a eBay subforum discussion, but anyway...
To be honest, I think the best site would be to sell on Tradesy (I think the European equivalent is Vestiaire Collective). I've used it and I've had no issues with it whatsoever.
If you want absolutely no trouble, sell on
Fashionphile or Yoogi's Closet. Be aware, however, that you will lose a sizeable chunk of money. I've used both and have had no problems with either, although Yoogi's tends to be very picky about what they sell (and they tend to offer less than Fashionphile). I've actually gone through this route more because I had a horrible buyer and it just completely turned me off from selling to anyone personally.
I've never used it, but eBay is another good option, and I believe you get the most money from there. I just don't like it because I'm terrified of getting scammed.
There is also Poshmark, which a lot of people like, but I personally hate it. The buyers on there like to lowball a lot and with insulting offers (even worse than Fashionphile or Yoogi's). It's only a good site to unload used clothing and mid-tier brands like
Coach or Michael Kors.
And I know this is a real risky one and not everyone recommends this, but I've done craigslist local sales. I've always told buyers to pay cash upfront and that we meet inside a public area (and I will sit in the far back ,away from the entrance with my boyfriend). I only like this because I get the money right away.