Hi,
IMO,
As a domainer, I think you have listed your domain names in several marketplaces. If so, you can send your marketplace link of that domain to the interested buyer. It's better to exchange your domain through the marketplace.
You can also use escrow service from Escrow.com or Epik.com. They will complete the deal and you don't need to worry anything about your domain or the payment.
Another thing is, if you try to sell a domain outside of the marketplace, please be sure about the buyer isn't a fraud.
And, don't transfer the domain name before the payment is received while you are dealing outside the marketplaces and escrow services.