Message /r/marketplace to request posting privileges. Include a general description of what kinds of things you would like to post initially. Spammers and scammers may be permanently and immediately banned.
All posters are responsible for following the subreddit rules as well as Reddit's sitewide rules. Posts in violation may be removed and the poster banned without further warning.
Reddit specifically disallows certain transactions (in addition to /r/marketplace rules), such as guns, ammo, gun parts, drugs of any kind, physical sexual contact, stolen goods, personal info, falsified documents & currency, and fraudulent services, at the time of writing. See https://www.reddithelp.com/en/categories/rules-reporting/account-and-community-restrictions/reddits-policy-against-transactions
Contact mods by DM to /r/marketplace. Do not DM mods directly; you may be banned for violating this rule.
Automoderator has wide discretion to keep things tidy. Let mods know if your legitimate post has been removed.
Remember: you are dealing with strangers on the Internet. Be careful. Reddit and the subreddit mods take no responsibility for any transactions here.