Just had my MS70 silver gilt 1 BTC sell for 3.5 WBTC on 4k, not sure how to feel tbh. Listed at fairly high prices because I wasn't that motivated to sell but apparently someone really wanted one 
txid: https://etherscan.io/tx/0x5d2dea2ab81fc8db685a4646b9b8cc5bd777a97714a01e3ba9542285e86f1c4c
txid: https://etherscan.io/tx/0x5d2dea2ab81fc8db685a4646b9b8cc5bd777a97714a01e3ba9542285e86f1c4c
I'm just curious what made you willing to trust this site? Seems like you could take that 3.5 BTC and buy 2 more of those coins from here to sell on that marketplace. I know with a 250% premium on Casascius coins I would sell my entire collection. I feel like this might fall into the too good to be true category. The whole idea of sending them your coins in exchange for an NFT smells a little. I'm happy for you though. Were you ever worried you wouldn't get paid or that they would take your coin? I don't think I could have taken such a risk. Glad it paid off for you. It must have felt great seeing those coins hit your wallet. I hope others have a similar experience.
I would trust them. They have real $ backing them. They arent running off with the coins if thats your worry. (I dont know any of them personally). StockX also announced they will be doing the same with sneakers.