I am not sure if this explicitly is against the forum rules, but I would say that paying to get a thread bumped more then once every 24 hours would probably at the very least be frowned upon by the mods and could potentially lead to a ban (in theory at least).
It also appears that you are offering a spam service which has somewhat negative ethical considerations.
If done in the correct manner, it seems to be ethically sound and effectively no different from a normal signature campaign.
I take each client on an individual basis and have refused to provide for some services that clash with my own ethical beliefs, for example an unsecured IPO.
It would be one thing to have your accounts wear a signature pointing people to a specific thread but paying people to post on a thread is very similar to bumping a thread yourself. I would say that paying someone to bump it once per day should be fine but bumping it more then that is breaking a specific rule.
With a signature campaign people viewing your advertisement have the option to view a thread/website, purchase the product/service without posting on the thread.