How to cancel an unpaid item from a seller

[x3pink]

My sister in law accidentally used my account to make offers for an item that she wants to purchase. An offer was accepeted and I contact the seller that my bank account was insufficient at the moment due to personal reasons. Seller refuses to cancel and said I should of known about my account before purchasing. What can I do? What can be done due to lack of seller's understanding? If item isn't paid, will there also be a problem? All the seller tells me is that he or she will get charge for cancelling which I dont find why she or he would be.