Unhappy customer

On the 22nd of December i bought a second hand bike from this store for my son for Christmas, this is what he wanted. On Christmas he got a new bike as a present, so this one was not needed anymore. He never rode the one i bought. On 31 December i phoned the store to enquire about returning the bike, they said they do not do refunds but would buy it back from me. I thought this was quite unfair. The amount they offered was almost half of what i paid - their reasoning - they need to make a profit. They are making 100% profit on this while i have lost out. I am not happy about this. I understand it is not their problem that my son got another bike, but they could have refunded me the full amount, instead of taking so much back.