I've sold a couple of old bikes, but only to fund new ones. The last bike I built was done so using parts from an old bike. That bike had been upgraded and renewed so much that by the time decided I wanted a change only the frame remained of the original. I sold the frame and built the new bike from those parts, then decided I wanted a few upgrades. For every component I bought I sold the original component in order to minimise costs. I've just done a mental inventory and realised that of all the parts I transferred from the Cube onto the Cotic, only the crank spider, forks and gear shifters remain. So, yes, I suppose I always sell to make way for new, but bikes aren't so much a whole as a tidal flow of parts and accessories.