ebay authentication.

Hi, I am trying to authenticate users on my site by authenticating them against ebay. Dont ask me why, I just have to. :) I need to know wether the user entering my site is a valid ebay user or no. The only way I thought I could validate them is by checking there login name and password. I thought of posting the username password directly to ebays sign in form and processing the output, but that doesnt seem to be working. Could someone suggest an alternate method? Warm Regards, Kunal Jhunjhunwala "Minds think with ideas, not information. No amount of data, bandwidth, or pocessing power can substitute for inspired thought," - Clifford Stoll