Q: this apple id has not yet been used with the app store site

Did you follow the instructions in the message?

Did the seller give you the original install DVD?

Which OS version is on it?

Before you can download anything from the app store, you must have an Apple ID. You can create one without using a credit card:

Create an iTunes Store, App Store, or iBooks Store account without a credit card or other payment method - Apple Suppor…

If it is an old iMac (how old is it?) and has Yosemite on it, post back after you create the Apple ID so you can try the app store - if it works, fine. If not, the seller did not do what he was supposed to do:

the seller must remove (erase)  the OS downloaded from the app store because the license is not transferable - if the seller did not do this, then you can't reinstall the OS or update apps.