there's always frames that i regret selling, but back in the day i had to sell the old bike/frame to help pay for the next one. i topped out at 33 bikes in total, when i got made redundant in 2014 i started selling some of the fleet, frankly i don't miss any of those bikes, that said the ones i can't bring myself to sell are ones i have had painted, mainly because i know i have put strong money in to having that paint done, also because those are the bikes that got XT/XTR builds etc.
so the more generic, unrestored stuff that's not sentimental got sold, and with those ones you tend to not lose too much money as most of the time you can sell those for similar money to what they cost you. what i would say is consider selling frames only if you have good componentry, say for example you have a full m900 xtr group, you could just box that up and stick it in the loft, sell the frame it was hung on if you're not fussed about that frame, then if by chance a cool frame pops up in a couple of years then you have the parts to build it up. frames move around a lot, but componentry can be hard to find and expensive
what i will say is i barely ride my retro stuff these days, the modern stuff is better for everything i want to do on a bike so the retro stuff gets used as show off stuff on the weekly 4 mile round trip to the brewery with some other bike mates, so truth be told i wouldn't miss riding them if they were all gone but i'd miss just having them, i quite often have them in the shop and they can be useful sales tools when showing an older rider the difference between new and old geometry etc.
