AITA for not wanting to return someone’s money

I recently sold something on Facebook marketplace I received from a family member moving it was brand new in the box and the person I sold it to asked how long I had it my family member moved last year so I guessed 2 years supposedly the item was manufactured in 2017 it was brand new in the seal so I wouldn’t know but it it expires in 2 years i sold it for 80$ and original price is 120 and there saying they overpaid for it but I told them it was already 40$ under market value and there trying to demand 30$ back and I don’t have much money but for what they gave me the item I sold was a nest fire detector I just wanna know if I should give them the money back or if it’s ok