Well lets talk about the confusion.
Well payments and subscriptions are different things so lets get that out of the way so you can stop being confused.
As far as payments you can setup the payment options and under edit categories for each category you can set a payment amount to place an ad in that category. You of course control what groups are setup to pay in admin edit usergroups.
Now say for instance you want to create groups that dont have to pay everytime like regular users thats where subscriptions come in. If you have vbulletin I would suggest you use the
vb subscription system because its been around alot longer than ours and is more feature rich as in its going to add features to both vbulletin and classifieds basically. You can sell access to forum and classifieds only content etc in one package. If not then use the subscription thing in classifieds it will work for classifeds. Simply put a subscription can elevate a user into another usergroup where you can set different permissions like for instance access to not have to pay each time they place an ad for the duration of there subscription.
I hope that clarifies some things.