Let’s say I’m selling you a book B and accepting a crypto payment. What if you sent me your crypto C trusting me, but I exit-scammed, vanishing without sending you B you’re trying to buy? That’d be bad. But what if I sent you B first, trusting you’ll send me C as soon as you receive B? Now you could cheat and vanish without paying. That’d be bad too.

To prevent any of those things from happening, there are a few methods. One is a 2-of-3 escrow service. Another is 2-of-2. Both based on multisig. A simplified example follows.

The book costs you 100€. You’ll send, say, 200€ to address A controlled by both you and me via multi-signature. I too will send 100€ to A. Now Wallet A has 300€. When 2 persons (you and I) sign, there will be a 2-output transaction from A to you (100€) and to me (200€), but any single person can’t move fund from A. That’s multisig.

Now I must send you the book in a good condition, because I don’t want to lose my 100€. So I’ll act carefully and honestly, and sign when I ship the book. You too will be willing to sign when you receive the book, because otherwise you can’t retrieve your 100€ (you deposited 200, when the book only costs 100). Sometimes an unexpected accident may happen, but usually something like this will work pretty well. This is one way how a P2P platform works (not very accurate, but I hope you get the idea).