well i have 10.51 now, i did as you suggested and deleted all ebay cookies - my goodness they serve an awful lot of them!
anyway i still can't sign in - if i use the WRONG username/password combination then i get a message saying:
Your user ID or password is incorrect.
however if i use the correct combination, i just get returned to the log in page (and i haven't been logged in)
i really hope it gets sorted soon. i'm sure ebay don't test their site with opera, but opera should test ebay with their browser!