I know that selling a partyhat in the general store will give you 1gp, but after i sell it for 1gp will i be able to buy back with 1 or maybe for 3gp ? or maybe i will have to pay 80m (purple partyhat price) for it ?

You have to buy back for ge price.

Not that I don't trust you, but these type of methods (with other items also) are used badly. Trading items between accounts or selling them for real life money is against Jagex rules.

You can't sell partyhats to general stores.