So, assuming the 2nd generation rMBP comes out next year, what price do you think the current rMBP would sell for out of hand? (I'd like to sell mine and buy the upgrade for performance reasons.)
Yes, depending on location I'd bet that you'd recover 75-100% of retail for a year old version in excellent to perfect condition. Note that 100% of retail means that you're taking the hit on taxes (and the buyer is saving on taxes). In high tax jurisdictions, 100% recovery of original retail could happen.