I just always check the physical size of the loops when I'm looking at them. Medium only has 6 bumps on the piece with the loop, while Large has i think 9. Either way, not a good excuse to deny a return. I've only had positive experiences on Mercari. I ordered a Pure Platinum/Black for $15, seller said it was perfect, I got it, and it was so discolored I thought I was sent a spruce fog or something. Seriously, the whole face was visibly green from discoloration. I opened a case, and within minutes Mercari sided with me, refunded both me and the seller, and let me keep the band. I ended up giving the band to a friend and buying another in new condition off OfferUp, but wow, not a bad deal. Oh yeah, also ordered a woven nylon that got lost in transit and they refunded both me and the seller once again. The band arrived maybe 3 weeks later, but Mercari still let me keep both the band and money.