I bought a key to use your service. if I change my ID to ebay I will lose your service or not?

The key depends on your eBay user id.
If you create a new eBay user id while keeping the old, you can use the same key for both eBay user id (just activate the software using your primary eBay user id, then re-login using your other eBay user id).
However, if you change your primary eBay user id, your key will not match.