Sounds dodgey to me! New user on TradeMe with no feedback is always a sign to be careful.
Being a Compaq laptop it would have originaly been supplied with one of the Windows Vista versions (most likely Home Basic as you said).
Why would someone remove the COA sticker from the bottom?
If they did not supply you with a COA sticker or original box with product key for Windows Vista Ultimate it is most likely a pirated version.
Probably using the OEM BIOS emulator crack or similar.
You can use a Vista DVD to install any version of Vista you like. It will automatically install the correct version once you put in your product key or you can install any version without a product key and it will run for 30 days.