Hand made, exotic to the BMX world. Highly desirable to BMX collectors of 20" BMX bikes. Very rare in really good original condition.
I sold a hashed 1979 model two years ago with very few original parts for $350 in 30 minutes when I tried to sell it. I undersold it and could have got more if I tried.
https://thecabe.com/forum/threads/schwinn-sting-what-have-we-here.215884/
With the BMX market blowing up right now, and the condition that one is in, I would say that one is at the top end. Retail, with a little time and patience, I wouldn't be surprised if that was a $3500 bike.
But, you'll need to jump into the BMX crowd. The CABE is not the best place although there are a few here that really go for them.
No telling, but I'd pay $800, maybe a thou on a wholesale find like this.
If he asks for an offer, I'd start at $500 and judge his response. He may know what's up.