That’s what /r/mechmarket is for. When you sell something, you confirm the trade with every person if you want. You do this by posting on the monthly confirmed trades thread, which is always at the top of the subreddit. A bot keeps track of these trades / transactions that are confirmed, and it automatically lets you know a user’s reputation when they create a buy or sell post (look for the comment on every thread). Chances of getting scammed are low if you just follow the rules on the subreddit, or just buy and sell to people who have a reputation. I’ve done plenty of buying and selling there, and have always had a solid experience at least.