User Activity

Several times I have had invoices sent from sellers and the Ebay Cart gives a different amount and will not update or cancel. Why is this happening and how to FIX IT?
A seller offered a 2nd chance buy it now item after I lost auction. I took up the offer and paid for the item. Days later seller cancelled item and refunded the money paid after reauctioning the item on Ebay.Can anything be done to stop this bad prac...