I usually feel pretty good.
When I sell a keyboard it is to make (back) some money and clear out the clutter. If I sell it at break-even, at least I have opened up some (literal and financial) space. A profit is good and a loss is bad, but, either way, I am moving on.
There have only been a couple of occasions when I regretted selling one right away (and once I made a profit and replaced it quickly for cheap) but several occasions when the price went up significantly after a year or 2 and I wished that I had waited.