someone bought a second hand bike off me it was 2 years old they come to pick it up and all was good they liked it the next day they texted me saying the think something is wrong with the gear cable  and they will replace it now after 10 days they are saying they are taking it to the bike shop to see whats wrong with it and if its to expencive to fix they will bring it back for a refund. its a 2 year old bike second hand and they have now made alterations to the bike. and i entitled to give a refund? can anyone help me with this please.