By "they" I take it you mean eBay. Seller can't charge you anything, all payments are made to eBay.
We don't know exactly what's happened here, but eBay's procedures are automated and its unlikely in the extreme that eBay would invoice you for anything other than an accepted offer, if this was th eoutcome of the sale.
If you're certain that this is what happened youi would need to contact customer support. Do first check your facts carefully; almost all complaints of this sort prove to be a mistake or misunderstanding by the buyer. For example, did you realise than any offer you make does not include the delivery charge? Have you taken account of the buyer protection fee, if payable? Or could you have accidentally clicked on a "buy now" option instead of making an offer?
