I recently got this repair kit from the gachapon and I don't know how much it's worth.
Would I be better off just getting a ruined ribbon and fixing it myself to sell?
Well, ruined ribbons are the expensive things, so if you have the money to buy one, or enjoy spamming Barri, I say you keep it.
If not, they usually sell for 800k-2m, depending on the seller. I'd shoot for around 1.2m...