Selling him would be a stupid idea.
The guy is 36/37 yrs old... if he decides to go, we should allow him.. he's been fulfilling his functions very well while he remained with us.
And regardless, Marotta always says it's the players' decision...
If he wants to go and be the starter and an important player for a Serie A team, let him go. But we must really worry and go for a reliable 2nd keeper, given Buffon's age and history of injuries. Leali and Fiorillo aren't adequate for now.
It seems we'd go for Sorrentino, but I don't really know how good he is.