I would say Ebay. It's where you are most likely to attract someone willing to buy old clothes. Bear in mind it may take a few days as I tried to sell an old phone and I waited 1 week and someone only bought it within 1 hour of the auction closing. There's also gumtree but people will waste your time on there and I'm not sure if it's for people outside the UK.
Good luck!
eBay or poshmark. eBay is probably better because everyone knows about it.
