A bump is deemed as an irrelevant or insignificant post used to push your thread to the top of the page. Some people think they're slick by posting stuff like "Back online and I'm still selling" or replying to a vouch with "Thanks for the business" within that 24 hour period. This will no longer fly. However, things like a deal annoucment, the tagging of a minion to change a price, or replying to buyer who has a question or complaint are all significant posts. In all honesty, you guys should be able to differentiate between the 2 on your own.
This is to reduce spam and other useless posts.