WTP – my understanding of the practice has always been that they are selling the chametz on the pots and NOT the pots themselves, since selling the pots would required tevila as you pointed out.
My understanding is that it’s a chumra since there is (hopefully) not a kezayis of chometz in your pots. M’ikar hadin, while one is not allowed to eat any amount of chametz on Pesach, you are allowed to have chametz in your possessions if it’s less than a certain amount. I used to think that amount was a k’zayis but most people I know seem to say that it has to be less than the size of a cheerio, so I think I was mistaken about the kezayis. In any case, you would not have chametz in the amount of acheerio on your pots either.
However, there are people who are makpid not to have ANY amount of chametz in their possessions on Pesach. (either because they are being machmir or because they are going by another opinion). Presumably, these are the people who are selling the chametz on their pots.
I don’t think it’s the b’liah they are selling – I think it’s any grease that may be left on their pots if they didn’t clean them super well.
But again, everyone should check with their LOR.
Personally, in my home we never sold the chometz on the keilim, but now that I sell my own chametz, I think I remember that it’s automatically included on the form, so I think I do end up doing it anyhow, but I’m not sure.