Twice now I've pasted in eBay item numbers for bidding and get an "Item number appears to be incorrect" message, not allowing me to bid. The item numbers were NOT incorrect.

Hello Steven,
Thank you for your question about entering item numbers. Please ensure that when you paste the item number, no extraneous characters (such as spaces) are entered.
Thank you for taking the time to write in. Please let us know if you have any other questions. We are happy to help.