It sounds like he may not have the boost gauge hooked up correctly. If it was reading 10 psi of vacuum at idle that would be one thing, but the car should not be reading positive boost at idle. If you're going to get the car, for your reference, you want to hook the reference line up to the manifold or directly off of one of the vacuum lines that comes directly off of the manifold with a T connector so you can read absolute manifold pressure.

 

I was wondering if the boost gauge was hooked up to manifold vacuum, since manifold vacuum drops when the throttle plates open. Is that what you're getting at 95? But, that's not where the turbo boost gauge wants to hook up?

 

A boost gauge should be connected to measure absolute manifold pressure, both vacuum and positive boost. He said the gauge was reading 10 psi at idle and then he revved it and it went to 0. 10 psi vacuum at idle sounds feasible (30 in. Hg is about max, which is 14.7 psi). That said, the gauge should not be reading positive 10 psi. Usually it's pretty tough to pull positive boost with just revving, it usually takes more load on the motor in order to spin the turbo enough to produce positive boost. If the gauge is under the hood it would be pretty tough to see if it's actually registering positive boost while driving. I didn't see the gauge, so I can't say if it looks like it would read positive boost.