I have been using StockX for years, since their very beginning, as you mentioned it was first sneakers, then other things also.
My experience with Lego and also my opinion is, it's a good place to buy, but not too good to sell, mainly due to its prices, fees (fee + tax + shipping), but couple times I found that it's cheaper to buy there. Also the competition seems to be low compared to other market places.
I do have to add that their customer service is one of the worst, maybe that's the "not good" part. But if nothing goes wrong (everything is always perfect right!), then you wouldn't need to deal with customer service. =)