I'm sure I'm repeating stuff, but clearly state PAYPAL only, United States only, 25+ feedback only. 25 is a pretty decent amount, you can go more or less depending on how careful you want to be.
The ultimate security is an escrow service- this gurantees both the buyer and seller security. Here is a statement from eBay themselves (short of it- only use escrow.com, DO NOT use another service suggested by the buyer, for example).
"There are fraudulent escrow services, so use caution if a seller suggests using a service other than
www.escrow.com.
If your trading partner insists that you use a specific escrow service and will not accept
www.escrow.com (or an eBay approved international escrow service for international transactions) as an alternative, please decline the transaction and report the seller or buyer to eBay: "