It's funny that with all the crappy payment systems that have been popping up, it's less safe to perform this sort of transaction than it's ever been. A cashier's check can be forged, so a wary buyer may not accept it. What you can do is ask your bank if they have a telephone number for check verification.

Wire transfer is also good, especially if your bank allows you to order it up from a web interface. That way, you can wire it on the spot and it will be in his account by the end of the day. But three disadvantages. First is that it generally costs about $35, depending on your bank and how much you have on deposit. Second is that the wire system is relatively slow. So if you conclude the deal and order the wire, you may be hanging around for a couple of hours until it can be verified in his account. Biggest problem is that the seller has to reveal his account information, which can open him to fraud. So again, a wary buyer may not like it.

I actually find that some of the online payment systems work better than traditional banking products. If you load the cash into your paypal account, you can send it to him nearly instantaneously. Just check the transaction limits imposed by whatever system you use.