Each registrar is different. Inside the registrar it will give you the push option. The only registrar that allows a "pull" is dynadot. The others only allow you to push, otherwise you will be pulling names out of everyone's accounts and that wouldn't be a good thing
Nuances could be:
1.) Buyer charging back the money they paid you.
2.) Backing out after agreeing to purchase a name
3.) Using a fake credit card
Some good things to look at is a verified member seal next to a users name. This means their address details have been verified by DNF and gives a little more security. Next, is the users iTrader feedback rating. This is similar to eBay where others leave feedback from previous transactions. You can also look at how long the user has been a member. The list goes on and on.
Hope this helps.