So, I've had an idea but I'm not sure how valid it is or if people would join in on it. But is it allowed to have a thing where people collaborate on a shop or something to all give money to me and if they raise the money together that I say, I release a set to the public for all to download? Or is that against some rule somewhere as a multi sale or something?

do you mean people put tc in 1 set and all of them get it and be able to sell it ?
if yes its ofcoursly against the rules

actually i dont know if the art board cares but i do know that it is against the rule in art/textures board and there was one a long time ago (sometime in 2011) there was a person selling free textures and he actually got infracted :/

Well free textures are allowed so there's no problem with that aspect.
What I can see there possibly being problems with is the collaborative buying part. As far as I can see there isn't any specific rule against it so if three people all put in 1/3 of the cost X with the agreement that when it's funded the set become free for everyone then that's fine. It's the same as one person buying a set in the market and then making it free. Where issues might arise is what sort of conditions are attached to the paying. What happens if users put in money but the goal is never reached? Do you keep the money in a funds account until it's all raised or will you be using it as soon as you get it for other stuff and just keep a record of payments? I guess as long as that sort of thing is clear and contributors are aware and agree to the terms then it should be fine (but still too messy to be worth it imo). But like... if you want to do something like this why not just straight up release it for free?

Well I want to offer the set to everyone but I'd like at least a tid bit of tc for the 100k set I have.