USPS Money order is a good alternative to cash. They are fairly difficult to counterfeit.
I always bring two copies of a simple firearms bill of sale, check the buyer's ID to make sure they are a resident of the State, and then have them sign the firearms bill of sale. They sign to the fact that they are not a felon and are legally allowed to own the firearm. They also include there DL number and/or address on the BOS.
It really protects both parties and if one of the parties does not want to do this, there is very good reason to be suspicious as the buyer or the seller.
Both the seller and buyer's full information should be on the BOS. The last thing you want to do as a buyer is to purchase a stolen firearm. At least with documentation, you can provide the sellers information if the police come for some reason.
As a seller, you want to have proof that you sold the firearm in case it turns up at a crime scene.
As a buyer, you can bring the BOS for the transaction. If the seller isn't that prepared, he might appreciate your effort and feel more confident that he is selling to a "good guy."
There are lot's of free online forms available...
http://www.google.com/imgres?q=firearms+bill+of+sale&hl=en&sa=X&tbo=d&biw=1920&bih=951&tbm=isch&tbnid=qTJjyZF9zoOUBM:&imgrefurl=http://utah-concealed-carry-permit.blogspot.com/2010/07/sample-firearm-bill-of-sale.html&docid=WHoxU0YJKwwFLM&imgurl=http://2.bp.blogspot.com/_bVfGTWHA7Uc/TFJNdkfzR0I/AAAAAAAAAJM/XgaIY42c9Tk/s1600/Firearm%252BBill%252Bof%252BSale.jpg&w=1179&h=1600&ei=sZzbUI-MG9DOqAGiuoGQCA&zoom=1&iact=rc&dur=469&sig=109595912719886163655&page=1&tbnh=144&tbnw=106&start=0&ndsp=70&ved=1t:429,r:0,s:0,i:90&tx=65&ty=68