I agree with the opinion of Jet Cash.
Imagine that 100-1000 such bots will work at the same time, new offers of services will simply be unnoticed. Maybe you bump topic every 36-48 hours?
unfortunately, forums aren't the best places for selling things especially when a forum is as popular as this one. What we tend to have is that one user will sell a magnitude of different types of goods/services, and have like 5/6 threads all of which they can bump. Possibly limiting the threads they can bump per 24 hours could be an option to look at.