They do have other players they can sell, you know?
While this is true, notably Rakitic, that's not necessarily the central argument being made as only Messi, Suarez and Neymar could theoretically generate the cash inflow Barca would require to balance the books (and extend Messi) regardless.
The point is, Messi hasn't signed an extension with the club yet. If that persists beyond this next transfer window then in summer 2018 he leaves on a free and Barcelona miss out on the largest transfer fee in the history of the sport, plus end up with egg on their faces for letting it happen.
So far, Messi hasn't been extended and the prevailing theory is that Blaugrana had to choose between extending Neymar and extending Messi and they picked the former, now the highest paid star in their team and indeed in Europe.
What does that mean for City? IMO, Messi will be extended, but if he's not, then he'll most likely be a Blue.