I guess it depends on how comfortable you feel with who you're doing business with. I usually get payment first and then ship. I consider myself a trustworthy person and everyone here who has dealt with me has not been led astray. But you do have to be careful.
I typically take care of all financial transactions through PayPal. It's a wonderful service. And they have tools in place to protect you if a deal goes bad.
Just make sure when you're dealing with someone to get as much info as you can. Their name, address, phone #. And don't be afraid to check it out beforehand. Get a lot of pictures of the merchandise as well as a complete description so there are no surprises when it arrives.
Those will just get you started. I have been lucky and have had nothing but great transactions to this point. There have been a few others who have had troubles in the past. They should be able to give you some good advice, too.