What can I expect a seller to demand if I want his car looked at by my

[Barnabas]

mechanic before I consider purchasing? I'm sure I don't want to buy a used car without having a mechanic to first check it over. I will have to drive off with his car and having a mechanic to look at the car could take at least a day, maybe less if I prearrange a visit. What I am not sure about is - what can I expect the seller will want as security?