Trainers

You buy the sneakers, the seller sends them to stockx for verification (legit, unworn, etc) then SX send them to you.
You end up paying for the stockx bit on top of the purchase, plus delivery.

I've not actually bought or sold through them (foolishly wore the trainer's and then noticed they are going for triple what I paid, ho hum)

I've used them for something my daughter wanted, paid the lowest offer price, then paid the fees, postage and import taxes, trainers were £68 ish, ended up paying about another £26 on top of that, took about 10 days to get them from bidding. They were brand new and still worked out cheaper than UK retail if they had been available.
Sizes are american on the site.

You buy the sneakers, the seller sends them to stockx for verification (legit, unworn, etc) then SX don't send them to you because they are fake.

That was my only buying attempt.