I'm sympathetic to these issues, both as a buyer and as a seller.
There are a few shops near me that I would not let anywhere near my aircraft and, if that meant losing a buyer, so be it. My view is that, while I own the aircraft and am responsible for paying its bills and for its condition, I am entitled to be very fussy about anyone touching or flying it.
On the other hand, no buyer should be expected to make or pressured into a purchase decision without being comfortable that an independent and trusted expert has assessed and reported on its condition. After all, it's caveat emptor in most aircraft sales/purchases.
A prebuy inspection must be subject to the seller's approval and, if the seller's conditions as to location, shop or the extent of the inspection create a situation where the buyer cannot achieve what he/she wants from the inspection, the buyer should simply walk away. That said, the buyer should understand, and in some cases take comfort from, a seller who is maintenance focussed and concerned about what an unknown party might do to his/her aircraft - it swings both ways as one also has to be very careful about a seller trying to hide something.
I've seen terms in purchase contracts about the buyer returning the aircraft to the seller in no worse condition than prior to the prebuy inspection (if the deal does not go ahead). It's a little vague but captures the spirit. There are also typical terms around the prebuy shop being satisfactory to both parties.