I’ve never had problems on here, not that that guarantees anything. But basically, if someone has an active profile, posts their music, etc., then I’m usually comfortable buying from them. The potential for public shaming is hopefully great enough to prevent them from being shady.

You can also ask the seller for references to other people they’ve had transactions with.

Pay through PayPal (not as a gift) so you have buyer protection.

The chances of a decently packed item breaking in transit are very slim.