First person who shows the money gets the bike. That always worked best for me. Lots of people call they'll for sure buy it and then don't show up.
If a deal sounds exceptionally good and the seller seems trustworthy you can buy it unseen and send a deposit (10% or work it out with the seller) by ATM. Then it's officially sold. In this case a low risk transaction.
Then again I'll never forget how I bought my second bike in California in 1988. (My first one was a CB360, it came with a nail file for the points...) I saw an ad for a one-year-old Nighthawk 450 for $700 - it must have been new somewhere around $2500 - and I was the first to call. It was on a Monday and I didn't have time until Saturday, but the owner, an old man, told me that he would wait until then. On Saturday I went to see the bike, it was good as new, and thanked the guy for waiting. He said he was glad that I showed up because all week long people were calling to buy he bike, sight unseen. He took their phone numbers in case I didn't show up, but he said since I called first, I should get the chance to buy it.
Then he told me story why he's selling it: His grandson needed a bike to get to work and promised to pay him back with monthly payments, so he bought it for him, but kept it in his name. After that the grandson didn't stay in contact with him, didn't make payments, and soon after quit the job. Realising he'd been taken advantage of he decided to teach his grandson a lesson; he called him to stop by, took the key of the bike and gave him a ride home in his car. Then he put an ad in the paper to sell it quick way below its actual value - and I was the lucky guy who spotted the ad first and got it. It was a great little bike and I did lots of trips with it.