^

I actually agree with ll2 that a trade-in is often the best way to go (if you're planning on getting a different bike) if the bike you're selling is fairly expensive (500K+)- you'll get a bit less, but the stress factor is so much lower. If we're talking about a scooter, I would allow a test-ride without much more than ID- I'm thinking of this on a personal level and I'm just leery of strangers riding my big/heavy/powerful bike without knowing if they can handle it (or knowing if they're just test-pilots who want to try it out for free).

Use a buy-back agreement. The buyer inspects the the bike, if all is well, he buys it. At that time, you sign an agreement with the buyer that you will buy the bike back, no questions asked, within 30mins or 1 hour or some set time (provided there is no damage - take photos). Both parties in the transaction are equal, you are each in possession of the same value at all times. I have used this in the US, but never in Thailand. It works wonders to separate the serious buyers from the lookers.

Another suggestion. Sell it "mob-handed". Have at least 5 of your mates there and at least 2 of them should have faster bikes than the one you are selling.