What responsibilties do I have as a seller selling something online

Viberduo

Put in debt section as not a consumer thing and it makes me in debt!

I sold a electrical item via a online website(not ebay) buyer received and claims item is used(though later seems to backtrack that) and significantly not as described.

All that is wrong with it is the box I sent it in was not the original box though for the same item so the serials dont match and the box is a bit bashed about so they think its stolen apparantly so demanded a refund.

I didnt refuse but tried explaining that it was new just I had 2 of same item but when I moved house a few months ago I used the box for packing, and couldnt find the box for the new one just the old one and apologised for not making this clear, I thought to myself its only a box and does not affect the item itself as most people bin the boxes anyway.

They kept asking me questions like where did I get item from, can I prove I bought it etc then said they will get the police involved.

Aside from being annoyed and quite upset over this, they opened a paypal dispute making me unable to use the money(or my account) till this is sorted.

I am on benefits so if they win I will be out of pocket, they also said they can meet me to hand over in person but then I have no proof of a refund.

What legal rights do I have?