A middleman is a trusted third party in a person-to-person trade. The seller gives the skin to the middleman, the buyer pays the seller, and the middleman then passes the skin to the buyer. The idea is that neither side can run off with both the money and the item. Middlemen were common in forum and Discord trading before marketplaces automated the job.
Today the same role is played by marketplace escrow. A site holds the buyer's money while its bot holds or tracks the item, then releases both when the trade completes. That is a middleman with a company behind it and a record you can check. A human middleman is only as good as your reason to trust them.
Is using a middleman safe?
Only if you chose them yourself and can verify who they are. The most common scam is the fake middleman: the other trader suggests a "trusted" person, who is really the scammer's second account. You hand the skin to the middleman and both of them vanish. Another version impersonates a known community middleman with a copied profile name and avatar.
Check the middleman's real profile through their known link, not a link the other trader sends. Compare account age, Steam level and trade history. Be suspicious if anyone pressures you to use a specific person. With CS2 trade protection, items received in a trade are protected for 7 days and you can reverse your trades from your Trade History page in that window, but reversing undoes all your protected trades from the last 7 days and locks your account out of trading and the Market for 30 days, so it is a backstop rather than a plan. Steam also says middlemen should not be needed and can scam you by impersonating another trader or keeping what you give them.
When should I use a marketplace instead of a middleman?
Almost always. A marketplace with escrow does the middleman job automatically, keeps records, and has a stated policy for sales that fall inside the 7-day trade protection window. A human middleman cannot offer any of that. The extra cost is the site's fee, which you can compare at /marketplaces/fees/. For high-value items, the fee is usually cheaper than the risk.
If you must trade person to person, use a marketplace's peer-to-peer listing rather than a stranger's middleman, and never move an item first on the promise that money is coming.