So if I only have one item left in stock, I should not be allowed to list it on two seperate advertising formats?

Correct, because they are the rules of ebay.

If you have only one house to sell and a the Real Estate agents terms are only list with them or not at all,

well you can't list with them and need to go elsewhere. (If you want to list with several)

Same if I join a club, I respect and adhere to the rules no matter how silly they may be because I agreed to them, otherwise I can freely leave.

I am just wondering whether there are perhaps some other rules on ebay that you have decided to interpret in your own way that has contributed to your own downfall.