I'd start with the scrap value of stainless steel and tell the guy that you will not charge him for hauling them away.
My guess is this guy does not know the value of what he has, and is hoping for the best. He just wants to make some money off of something that's collecting dust and taking up space.
Hell, he may take $100 for the pair and think he got over on you.
If you get them for scrap price, and they turn out to be useless, you will still be able to recoup your investment.
He asked you to make an offer. Don't be in a hurry to give money away unnecessarily.
+1, this. he's not going to run away no matter how low you go. If he had that many buyers he would have named a price to beat.