Always use a trusted escrow, ableonion has a good one i use regularly and never had a problem

Be wary of too-good-to-be-true offers

Verify every marketplace links before using, ensure they are the official links.You can use reddit for that

never give your personal information to anyone to avoid blackmailing and scamming

be wary of vendors who tries to force their way into your trust with numerous messages and proofs. they are usually scammers

what works for me is to buy small amount first, and if the product is good, then buy more

some groups sells contacts of verified vendors, you will be lucky to find one