If stuff has been in the back of the closet for awhile, and I forgot that I had it, then it is probably time to sell. However, sometimes I find some gems going through the closet, and then I just get more stuff out. It's tough to find a balance.
For me I find I try and stay focused on power rangers and transformers but then you see those figures you don't collect and you buy one and then some how you have a entire series and no room to put it. It's all a matter of space really. If I had more room I'd have twice the collection but atm I do not. I also agree with the buyers remorse as a option. There have been many times where I've gone to a store and come out with something out of collecting and then later go. "Why the hell did I buy this?" Example black series boba fett. I don't collect Star Wars stuff but heh boba fett I own.
Nothing. I bought it cause I want it, so there's no profit motive and I'm blessed with plenty of space. Worst case I'll give away duplicates if I get a better sample. My friends have kids.