Where it goes wrong
Discord DMs from someone with three days of history. A payment that arrives and reverses. A "sent it" with no tracking. The common thread is that you can't tell who you're dealing with, and there's no record of the deal outside a chat you can't prove.
What to check before you deal
How long the seller has been around, and whether other people have dealt with them before. Whether the item is priced against what it actually sells for, not what it listed at. A clear handover: tracked post, or a meet in a public place, agreed before any money moves.
Why a members-only marketplace helps
When everyone on it is a paying member with a real account, the anonymous burner problem mostly goes away. Reviews only from people who've actually dealt with the seller. Prices next to live sold comps so you know what's fair. Messages kept on the platform so there's a record.
Minted's marketplace works like that for members, and browsing is open to anyone so buyers can see what's on offer before they join. Payments stay between the two people; Minted makes the introduction and keeps the receipts.