Don't sell the skin till you have caught the bear a traditional warning against acting on an assumption that may turn out to have been ill-founded. The origin of the saying is said to be found in a fable added to a collection of Aesop's fables compiled in the 1490s. It is recorded in English from the late 16th century (early versions have ‘lion’ or ‘beast’.
sell in May and go away, come back on St Leger's Day saying relating to the cycle of activity on the London Stock Exchange. May, shortly after the start of the financial year, was traditionally a busy time, but during the summer months trading was slack as Londoners (including stockbrokers) took their holiday breaks away from the capital. The full form of the saying refers to the classic St Leger horse race (see saint), taken as marking the end of the English summer social calendar.
sell one's soul (to the devil) do or be willing to do anything, no matter how wrong, in order to achieve one's objective. With allusion to the contract with the devil that certain people (most famously the German scholar Faust) were formerly believed to have made: the devil would grant them all their desires in this life, and would receive in return their souls for eternity.
See also sell by the candle, sell the pass at pass1, pass one's sell-by date at pass2.