Some data: I have been watching eBay pretty diligently over the last year thinking I might buy another 427 to build up over time. Service-era blocks represented and appearing in photos to be new (NOS) appear every so often and seem to be selling in the $3000-3500 range. Usable and undamaged service blocks seem to sell in the $1000-$2500 range, depending on how much boring range is left in them and how much testing has been done.
My opinion: Clearly, to buy that block as-is without inspecting it is risky. Assuming he has to disassemble to sell, I would think you would need to right to say no if you saw something you disliked upon inspection. Lots of risks here....so if it were me, I wouldn't risk much more than $1K, if that. Not to say that's what its worth, as everyone has different tolerance for risk on these things.
Knowing you can buy a fully-prepped 427 aluminum Pond block from guys like Keith (
see here) for around $5k (and Genesis cast blocks are less), that sorta sets the tone for what used 1972 blocks are worth....something considerably less. Maybe its just me, but after owning mine for 3 years, where a $40-60k replica build is concerned, I see no difference in $2k and $5k....that $3k is pittance where maximum performance and maximum mechanical reliability is concerned.