i would not say its spamming or overposting...
if you need an answer to something this board gets so busy that it is very easy to get lost somewhere on page 2 or 3 in a matter of minutes..so a bump will make it more likely to get the question answered
It is a problem because if we allowed it then everyone would be bumping all the time which would just drive more posts down faster. So anyone not bumping would be thrust down to page 12 because of people constantly bumping their own threads.
We have hundreds of new threads every day and tons of questions that people (for whatever reason) don't care to reply to. Imagine what a mess would be if everyone that wanted a reply faster started bumping their threads every day, every hour, or every minute... what a mess our forum would be!
If you post a question or an item for sale that has not received a reply it may simply be because people think the question is already sufficiently answered in other places or they just don't know the answer. In cases like this it may be okay to reply to your own thread with more detailed information that could help people help you, but simply replying to a thread saying, "bump" or "what, nobody has an answer" or "please buy my xxx" is not allowed.