I'm a seller here, but when I'm a buyer - I check if there is consistency in what he/she does.
If we're talking about arts - check if the shown examples are at the same level with already sold art.
So many times I've seen ppl promote with artwork on amazing level and then when commissioned creating just mediocre level art.
Also... Someone can be new here, but already known on some different platform.
Another thing is communication. I tend to trust more those who not always write "Yes" to everything I say. I had one guy always write me "my dear customer" and in the end he was using some general art creator (I'm not sure how to call it, it was not hand drawn - it was generated thru some program).
So.... Know what you want, do your research, don't be blinded by nice words.
Although... Fiverr is a platform that protects both buyer and seller so at least you don't have to be worried about scammers 🙂