already sold but got a higher offer

I sold a bracelet for $5 within about two minutes of posting, and immediately after someone else offered me $15. I said no because I feel like that's pretty unethical, and they increased their offer to $40. The person trying to offer me the additional money has already bought a pair of earrings from me as well. I'm afraid one or the other will leave me a bad review. What should I do? $35 is a lot of money but I don't feel right refunding the original person.