Sell, for sure. I've never been very big on buying things from Buycraft, even if I don't spend my own money on it; in fact, I've only bought ranks on 4 servers, ever, and 3 of those ranks have cost less than 5 dollars (VIP+, Boa, and Sorceror; not going to mention the server names). I find it much better to sell the cubits for a decent rate, as my goal is to prestige and get to Ninja rank again (Parkour), which costs 105,000 mana; that's roughly 85,000 more than I have currently, so I've got a long way to go.