One of the gentlemen at the Sovereign Press booth told me that the "Serenity" game book sold out the first day, all 500 copies. Not certain if they expected to sell out that quickly or not, but he seemed pleased by that level of sales.
I don't know if it is typical of GenCon, but there were numerous game products released in limited quantities at the convention, such as Mutants & Masterminds 2nd edition (300 copies, if I recall), Rich Burlew's Order of the Stick prequel (1,000 copies), etc.