I've bought a couple pieces off there, the easiest way to prevent getting scammed is to just check their profile page. If they have a bunch of other glass posted up and make comments on other glass you're probably good to go - especially if they've already sold another piece in their collection (ideal).
If instead the guy has only a single picture up and hasn't made a single comment across the rest of the site, you might want to look for another one just to be safe.
People also usually start personal vendettas against people who fuck them over (I saw one on nuggetry where the guy sent a pelican router instead of a pelican case with the bong lol) and post their info all over the internet so you could try google any/all information given to you about the seller (nuggetry name, paypal info, email, address, anything you can get) to see if someone's been hustled already.
Worst comes to worst I think you can cancel your paypal payment just so long as you call it glass art not a bong or anything like that.