Keeping backup equipment?

I run 6 systems, so yes I have spares. 

I think I usually tried to keep a small number of backup components which could work their way into an office system... a break in system. It’s a question of balance. But using for trade in is a great way of giving your dealer a way of discounting the piece you want to buy. This way he can show the manufacturer that he changed you full price, but give you a great deal by paying you too much for the trade in. 

I like to have a backup for certain components on hand.  I don't like to be without music at home, so if my integrated amp or my phono stage were to give out, I would need to start shopping (not a bad thing, of course).  But I keep a (B-stock) Yamaha A-S701 and a Schiit Mani on hand as backups.  They also serve as a good basis for comparison to whatever gear I have in my main setup.  

In addition, I try to keep my stereo spending as close to budget neutral as possible, so I sometimes sell something before deciding what to purchase to replace it.  With the backup gear, I can keep the music playing and take my time to make a good decision and find a good deal on the next component.

I keep some older components as backups. I hold on to any component that won't make much money when resold
Right now I have four amps that I upgraded stacked in my back room.