Apparently all you want as long as they do not interact:

This is from Ebay Rules FAQ...

Q. Is a seller allowed to have more than one registered eBay account?
A. Yes. However, there must NEVER be any interaction between the accounts in the areas of bidding or feedback, and all user information MUST be accurate and consistent.

As to shill bidding, of course it is illegal in a number of venues.

I would encourage all to go to Ebay Policies and then to "Shill bidding." There is a tutorial there that is pretty elementary but one question makes it pretty clear that if you know the person that has the listing you might want to be pretty careful about bidding on their auctions. I would also caution if you take the tutorial to not answer question 13...

Just one man's opinion..

Mark