I have just purchased my first vehicle on e-Bay and it was a much better and easier experience than I expected. I dare say that a small minority of people have bad experiences, but for me it was perfect.
The first thing to do is your homework. Check out the values depending on condition. Take a look at Glass's Guide and Parkers Guide to get a good idea of it's value. Ask the seller any questions relevant to the vehicle and check the paperwork is in order. Also, look at the sellers feedback record to see if they can be trusted. Beware of the dodgey ones, there are bad people in all walks of life regardless of their job or position. (Even Hitler's mother would love him.)
After that has been sorted out you should have the same experience as myself, a very good one. e-Bay is brilliant as long as everybody plays by the rules.