very very true. I just picked up another late 90's TDI. I've been wanting a block so I could rebuild it as a backup for my current TDI which is at 370K mileage. The seller wanted the upper bound of KBB (around $3K) for his whole car. While the car was rustfree, it was not "mint." I said I'm literally only buying this for the engine, so I'm offering a few hundred bucks. He got upset and I gave him my contact info. I knew no one was going to pay his number, let alone "good" or "fair" KBB prices. The demand here in L.A. for TDI's dropped to nil after Dieselgate.
After three months of no takers, he was getting desperate. He dipped to $2500, and $2100. He reached out to me to see if I'm interested in $1700, and I still turned it down. He didn't realize this TDI had non-CA emissions and it would take some work to register and inspect it here in CA. People were not interested in the TDI, even though he did a bunch of decent engine work on the TDI and put in something like $3000 in TDI parts on the vehicle in the last 2 years (receipts as proof).
Eventually I got it for my original asking price because he wanted it gone before he had to pay for another month of garage parking. Also, he couldn't get it started because of an IP issue and dead battery. He was upset because he put so much money in that car but didn't get any of that money back. That's just how it goes. You can toss so much money at a vehicle, but you probably won't get any of that back when you sell.
I had the TDI towed to my garage, and within 45 minutes I had boosted the battery, jumped the fuel solenoid, installed a clean fuel filter, and purged the air from the injector lines. Got it running.