I don't 100% agree with the No Demo, No buy as the best protection, there is several other ways to know if the seller is legit. After all not all items require a demo.
Bad Signs: Empty Profile, Full Perm Items with no TOS. Brand new account.
Good Signs: Payment Info on File, Profile of the seller has pic, info, groups and Picks, Has a Blog or Website, Inworld Store. Store Policy, Store Profile, Detailed or longer then 1 sentence Description on the Item.
The list can go on, There is no 100% guarantee, Common sense always helps.