My first experience of buying fish online was only 2-3weeks ago as the fish I wanted weren't in any of my lfs and I didn't want to order them in (I don't know why) so I bought them online from a mbuna breeder here in the UK. I'm so happy I did, the quality of the fish I received were 10/10.
I buy almost everything these days and have yet to run into trouble, 99% of things for sale online are from genuine people like you or me trying to sell their stuff. If buying from an individual ask for a contact number and call them, pick up on their vibe and try to evaluate their personality and make a decision.