I don't think there's too much of an issue, I think it's just that sites want to be abl eto trust their payment source and cryptocurrency is pretty new - they like what ha salways worked for them...
I've bought things with bitcoin from places like scan.co.uk and they've been pretty good with everything. There's the additional problem with bitcoin in that returns aren't as easy to handle (imo) and transaction fees might have to be returned too a sther'es no RTS (return to sender) script in eveyr transaction that can be signed and returned to the previous owner for free too...
It would be problematic for everyone if they would do it right now like what you have said the transaction fee and of course the irreversible transaction both party would lose some money if a transaction has been cancelled out or faced any trouble.
But I would also like to see some big Marketplace like Amazon to accept or use crypto as one of their payment option.
And it could be troublesome for some buyers if they would face some scam items I used to shop online but I always choose cash on delivery so if something is wrong on my item I wouldn't pay the item and the guy who deliver it to me would ship it back to the owner.
If it was made in crypto then every time that there would be a damage to my item then I couldn't get it back since I already paid for it.