Basically, if you don't have a ton of posts and say you won't go first people will regard that as a scam. There are official middle men who are trusted and assist in transactions, and you pay them a small percentage of the trade for their services to make sure neither party gets scammed.
Normally if someone has a lot of vouches, like 100-200+, then they are trusted and it's basically safe to trade with them; although using a middle man is still recommended.