This could be considered a non-compete clause under California law. Such clauses are not enforceable in California. They are enforceable in Connecticut (I've had this issue come up relative to California and Connecticut before). As I haven't read your contract, I can't answer definitively, but, assuming this is a non-compete, even if you are sued in Connecticut, and a Connecticut judgment is obtained against you if you do sell any art in the next 6 months, a California court will not enforce it. One way to help your case is to only sell art in California for the next 6 months. Realistically, though, it is unlikely the gallery is monitoring your sales, or would have any way of doing so.
The gallery established this clause to prevent people who see the work in the gallery from contacting the artist directly to purchase a work. The clauses are common. If you were to sell a work that was in the gallery, and the gallery found out, they could argue that they are entitled to the commission they would have received. It seems unlikely, however, that they would incur the time or the expense to do so.
If you'd like to discuss, please feel free to call. Jeff Gold Gold, Benes, LLP 1854 Bellmore Ave Bellmore, NY 11710 Telephone -516.512.6333 Email - Jgold@goldbenes.com
It might be an enforceable clause. It may depend on what law applies, California or Connecticut.
Such clauses are not uncommon. You often see them with real estate listings.
Probably, the provision does not prevent you from selling the artwork, it most likely provides that the gallery is entitled to a commission on any sales of the artwork they showed.
If you are really concerned, take the agreement to a local attorney.