First, it is not difficult to steal a long arm. All furnishings have been cleaned out of homes in broad day light. A moving van, a few movers and an ability to break into a home are all that is required.
If you decide to go see it, bring a friend as others have said - preferably the strong, threatening type of friend! Meet the person in front of a police station, if possible. Get to the police station early, go in and introduce yourself and tell them who you are meeting and why.
If you try it and like it, tell the seller that you will go to the bank to get cash to pay for it - and he/she goes with you. If it is legit, all those things will be negotiable.
If it isn't legit, then the seller will probably tell you that it is already sold.
Having said that, I purchased my pcquilter and HQ16 + frame from Craigslist in 2010. Everything went well. My DH (6'5") was with me and did the heavy lifting. He's not really threatening, but it was reassuring to have him there.