If a projectionist screws up his projectors and has to cancel a movie, all the customers in the house get a free movie next time they come back.

If a bartender pours a drink that the customer didn't order, it's "Bartender's Mistake"... the guy gets a free drink.

If a grocer advertises canned goods on sale at BOGO (Buy One, Get Oone) but he doesn't have enough stock on hand, he has to allow the customer to substitute goods of equal or greater value for the same price. (In some places, by law.)

So, if you agree to buy a camera at a set price but the seller can't deliver the agreed-upon goods, what should happen?

This guy posted the goods on Ebay but then made a face-to-face deal outside of the auction? Now he wants to give you a lens that you didn't ask for? That's his mistake, not yours. (Bartenders Mistake! )

He advertised goods for sale but didn't have stock on hand? That's just like the grocer's mistake.

If you ask me, the guy should sell you the 120mm lens in place of the 80mm for the same price.
Either that or he should throw in that Polaroid back for free.

Just like the theater manager, the bartender or the grocer, this guy made a mistake. It should be up to him to make things square with the customer.

(Oh! BTW! If the guy did successfully cancel the auction and sell that lens to you before a bidder put down money, wouldn't he have avoided some fees from Ebay?)