Another vote for “don’t sell” here!
I never understand why people sell a bike if they’re not using it- in five or even ten years you might just think on a sunny day that you’ll get it out of the shed and go for a ride and then maybe have another ten years of enjoying mountain biking. If you have that thought and have to buy another bike, it won’t happen.
I get the space issue but can’t you break it down and store it that way, in a bike bag maybe?
I totally get the dislike of all the faffing around with mountain biking- I pretty much only road ride or run these days as I can’t be arsed at all with the endless washing and the utterly ridiculous expense of replacing kit but I wouldn’t sell my mountain bike.
I’m not fit enough to race on the road bike but mountain bike races tend to be more casual and relaxed so I’m keeping the mountain bike to enter xc races on, but training on the road bike.