As anona says, it's in some dictionaries so I'll add the word. I don't think it's quite reached the common status. Most of the usage examples I can see are from specialist publications dealing with business, marketing, etc. So it will be classed as rare.
The earliest usage I saw was from
Inc. magazine in 2000:
Compared with many E-commerce sites, Mrs. Beasley's is actually quite simple... it doesn't upsell, a la Amazon ("Customers who bought the lemon cake also bought these products"), because Harris finds that tactic annoying.
I know you only asked for
upsell, blackrockrose, but I think you'd also like
upselling and
upsold. They're available in a package deal, for a limited time only.
A recent example of
upsold, from a
Globe and Mail piece on buying a car:
Whether leasing or buying, decide in advance what make, model and features are on the wish list, and don't be upsold on unnecessary features.