Going back to Cali and contracts class

Still alive: don't worry, folks. Tomorrow I get to go to a hearing (yay) and then this weekend I will be in the Bay Area visiting.

In the meantime, a question from a fellow knitter: what is the legal status of sock clubs that make you promise not to sell the club yarns for some extended period? Assume that the rules of the club clearly prohibit sale, but the seller made subsequent clarifying posts on a knitters' message board establishing that trading, even with non-members, is kosher. Assume an existing policy, applying to all yarns by the seller, of no sales for the purposes of profit (but sales are allowed to get rid of yarn that you don't like, although markups are deprecated). What is the import of the parol evidence rule here? The status of mixed cash/yarn trades has not been established (this is important because the pricing differential makes one-for-one trades unlikely).

My inclination is that 1) you're contractually obligated not to sell the yarn, but 2) the chances of actually being penalized are relatively low, as long as you are relatively inoffensive about doing so. Thoughts?