PrePurch by Existing A&P?

Thanks, all, for your insight and guidance on hangars, TBO, etc I've asked
lately. Now I have another one for you.

I know that you should always get a disinterested 3rd party A&P to do the
prepurchase, but, suppose I find an airplane for sale at my existing home
airport, and the current owner's A&P is also based there? If I intend to use
the same A&P for future annuals, what's the harm if he does the prepurchase,
especially if I tell him that up front?

Thanks; - or am I thinking too hard?
Carl