Just wanted to offer an update on this, in case anyone was curious or it’s helpful to anyone at some point. Back in late October I spoke with the seller again (who lives out of the country, is not a US citizen, and does not have access to email or fax) by phone, and he agreed that he would sign a new purchase and sale agreement at $30,000 at the office of a notary public local to him, if I found one who would assist us. FYI, it appears that in Germany their "Notar" or notary public are actually the closest equivalent to our title/closing companies, and this is not a status given to just anyone who files the paperwork and pays a small fee as they essentially are in most states in the US.
In addition to the substantially higher price (which still leaves $10k margin for me on my double-closing to my end Buyer), I also added the following language to my revised PSA, so that I'd have better recourse if the Seller tries to back out again, like Jaren Barnes suggested above in this thread:
"DEFAULT; DISPUTES: If for any reason other than failure of Seller to make Seller's title marketable after diligent effort, Seller fails, refuses or neglects to perform this Contract, Buyer may choose to seek damages or to seek specific performance, per this agreement. In a civil action to enforce this agreement, the prevailing party to the action shall be entitled to recover from the non-prevailing party reasonable attorneys' fees, costs and expenses."
As always, I'm not a lawyer and anyone might use the above language, or not, in any of their own agreements, at their own risk.
I also rewrote the agreement to have the Seller covering most of the closing costs now, instead of my doing so as I normally do with a Seller that’s not a complete pain in my... Unfortunately, though, as I was originally concerned about and as others such as Jason Wollbrink also called out above, after the Seller verbally agreed to the above approach, on the phone, and I found a Notar local to him who would assist us (within probably 3 or 4 days of speaking with the Seller), the Seller then ghosted me (and the Notar) for over a month.
At that point, like Charlie Brown who had the football pulled away again, I cursed myself for falling for it and for wasting my time, and was ready to just walk away, finally. Although my agreement with my Buyer, who's been incredibly patient throughout this, doesn't expire until the end of this month, I contacted him and gave him the option to cancel it early and get his EMD back, or wait for another month to see if the Seller happened to come out of coma and start responding. However, the Buyer declined to cancel and said he would still very much like to buy the property, so out of equal parts of shame for disappointing my Buyer and frustration at the Seller for lying about never receiving (or signing and returning along with handwritten letters) various documents, I decided to take one last swing at it. Last week, I FedEx'ed (again, no email, no fax) a package to the Seller with copies of our original fully executed PSA at $17.5k, his handwritten note which he returned with the PSA asking about next steps in the transaction process, and his hand-addressed/labeled envelope in which he mailed me these documents, along with a copy of the Notice of Claim that I filed with the county against the property, and a new letter that I wrote explaining that he needed to contact the Notar and sign the new PSA for $30k as we'd last agreed to on the phone, or I’d have to pursue all legal recourse to enforce our original agreement at $17.5k.
This morning I woke up to a signed (and notarized, this time) PSA in my inbox, from the notary public’s office! Still have to get actual closing docs redrawn up and executed, but at least now if he backs out, I could pursue specific performance of the contract, with the legal expenses coming out of his $30k. We’ll see what happens next.