I don’t see why it would matter, but I don’t want to lose the deal over the buyer’s financing.

If you still don’t pay, the tax buyer will foreclose and own the property. 

Then find "boots on the ground in SJ" who know the city ... in the interim, feel free to bounce any seriously considered deals off me.As a long-distance investor, I would strongly recommend you stay away from Indian Town (North end) .. there are a lot of "cheap" buildings there, but the prospects for the area are muted.

That's exactly what I did when faced with this issue, meaning I had a buyer under contract before I actually closed on my purchase.

I'm going to be honest so I can paint the picture as to where I am as a new investor: I've marketed for buyers, practiced calculating ARV numbers so see how that works, and have searched for properties. 

He says he already has a few buyers interested in buying it from him, as is, for substantially more the we are getting it for.

I know of one on Long Island that I use on the regular for my traditional buyer and seller listings, who is also skilled in double closings.

Is it out of the ordinary to ask to do a walk through with a contractor so that I can bring some concrete numbers to my buyers?

in my mind this is something to flip to someone who can actually close it and operate it.. and I am surprised an owner got this far with out vetting the OP as a real buyer.. 

I'm not saying it isn't a good deal, I'm saying how many potential buyers can fund a deal like that?