The basic sequence
The buyer funds the transaction through the agreed service. The domain is then transferred or otherwise placed under the conditions required by that service. Funds are released to the seller after the transaction requirements are satisfied.
The exact mechanics vary by provider, so the platform's current instructions control.
Why buyers use a marketplace
A reputable marketplace can combine payment processing, transfer coordination, account support, and transaction records in one flow. That can be safer and simpler than wiring money directly to an unknown seller.
Keep communication inside the official process
Verify domains, payment instructions, and transfer messages through the marketplace or service you chose. Be suspicious of last-minute requests to move the transaction off-platform or send funds to a new destination.