Hi Laozta6133.
One of the best ways to receive money without the buyer being able to take it back is
http://www.westernunion.com/. With that website, they'll send you the money and you can go pick it up at a local Right Aid or CVS (assuming they support WU). However, this has no buyer protection so more times than not people won't do this method unless you have a lot of rep. It is also pretty expensive.
As stated above, when using Paypal the main reason for concern about getting scammed is the fact that someone can "dispute" your payment (basically tell paypal that you lied) to get their money back. Paypal offers very little seller protection regarding online goods, so what you can do is get them to pay you via paypal, dispute it, and then cancel their dispute BEFORE you trade them.
When I sold my account for a high sum of money (1,100$) I used Kamitsuna's paypal method:
http://www.markeedragon.com/forums/ubbth...2/#Post1129021. Basically this turns the transaction from an online goods trade to a real life trade. In my case, I traded him a piece of paper with my secret password and question on it. The object that you are selling becomes tracked, and when it reaches its destination the buyer can't dispute it and say that he never got it.
Of course, another great way to ensure that you don't get scammed is to buy from reputable people. Trust Who is a very good program set up by Markee Dragon where people pay a small amount of cash and send in their ID to help prove they're legit. You can tell their reputation by the bottom left part of their post under "ratings". There, people who bought or sold things and are Trust Who vertified can give the person a review. Often times people also post reputation threads in their posts or signatures, so make sure you check these out as well.
I suggest always being careful when you sell; don't let people rush you into a deal, take your time to check out their posting history (and search their name on the scammer forums!). Trading becomes a lot more personal when you remove the fact that they can't hide behind a computer. Make them scan their ID, call you from a land line, even friend you on facebook. Good luck, I hope this helps!