If you have a seperate windows xp user account for each ebay/paypal account, theres no need to wipe cookies. Regarding ip changes for logins im not sure about that. I think it's ok to use the same ip to login to multiple acc's but i wouldnt take the risk incase 1 drops and then they all get connected.
If you are however accessing all the accounts via a single username on your pc then yes wipe cookies/change ip etc. | 
