AT550

 Just saw a used AT550 on ebay.  I wanted to ask a couple of questions.  I read some past posts that suggested that the AT550 wasn't good for self-tonometry.........why is this??  Sounds like you just look at something, push a button, and it takes the measurement.  Sounds fairly user-friendly.  Also, if I were to buy a used instrument, how much do you think it would cost?  How much are one of these new??  Thanks for your answers on this one.