Your concern is an understandable one, but consider a giveaway/promotion which requires the individual to type their username from the external site. This would be no different to that.
Alternatively a sig campaign which requires confirmation of payouts or even registration on the sig campaign boosts said product/service.
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These threads belong in games and rounds, away from the rest of the forum. The threads are specifically kept separate from, other traditional sales threads.
I'm not going to spam with one account on the thread, that is not what I'm offering. I'm offering posts from a plenitude of accounts. Paying someone to bump a thread with ten of their accounts is in practice no different to paying ten people to bump it once, so I'm slightly confused as to your concern in this regard.
You are right that using 10 accounts to bump is no different then using one account to bump. The question is how often the thread gets bumped. If a thread gets bumped (either paid, done by the OP or done by another account controlled by the OP) more then once every 24 hours then it is against the spirit of the marketplace rules. If a thread is only going to get bumped once per day (24 hours) then I don't see any issue, however your OP implies that you would be bumping more often then that (at least IMO)