The dictionary defines the verb "upsell" as such:
Main Entry: upsell
Definition: to try to persuade a customer to buy a more expensive item or to buy a related additional product at a discount
It's a fairly common word that gets used a lot in most retail trades.
But the dictionary does not list "downsell" as its opposite. However, it seems to me that the concept of upselling does have a clear opposite concept. i.e. Trying to persuade a customer to buy a cheaper item or fewer items.
Granted, there are fewer situations where this would be used, but I wonder why the word does not exist, and whether there is another word that could be used in its place.