I would go for your option #2.  Have the taxpayer estimate the FMV of each item, and prorate the sales price per item based on that.  And as you said, indicate on the 8594 that they were not agreed.

It would definitely be ideal to get the address and ID of the seller, but I would say if you can't get it, just file it without it (and make a note with the e-file that your client was unable to get it).  Maybe write "refused" or something like that on that section of the 8594.