I've thought about selling my PC just to build a new one for fun, and I honestly don't recommend it. What you'd be able to build with that money is most likely going to be worse in performance, you'd be really lucky to get something that performed similarly. You'd have t invest more money to get something better or probably even get something similar.
If you have enough money lying around, build an APU system or something for fun. Something that is different.
If you're gonna sell, read the rules; need photos with your username written on a piece of paper. Otherwise, not a place to get suggestions; therefore this thread should be moved.
Sell some parts, get some new components, for me I recently upgaded my GPU's and it was a revolution, I had the new tech smell I actually got to install new tech GIGGIDY.
If you're after the new PC feel, buy a new case/learn to sleeve wires/look into watercooling. There's a lot of ways to build without losing moolah, but if you decide to sell the system as a whole be prepared to take an actual loss.
For me personally when I get the itch to build, I will buy a small yet significant component and live vicariously through that, or I will re-arrange my loop or wiring routing as it lets me fiddle with my rig for a few hours and I'm happy