Topics get moved because they're in the wrong section. Most commonly are mechanical issues posted in General Chat, or Performance questions in the Yearly mechanical sections.
I'd venture a guess that your topic got moved because:
1. You're not selling the car to begin with.
2. It'* not a 3800.
3. It'* not an H body.
If we allow everyone on this Forum to post stuff they see for sale somewhere, the legitimate Sale Ads people post for superchargers, cars, and rims will be bumped so far down the list that the For Sale section would quickly become ineffective.
There are far too many topics moved lately to justify a PM each and every time. Typically with new members, a shadow topic is left so they can find their way, and a note is placed in the reply which explains the move. With veterans of the Forum, you will typically not see this. When you realize your topic is gone, you have a good idea to go looking for it, and how to do it quickly.
If your topic gets moved, ask yourself 'Was it in the right section? Does it fit where it is now as opposed to where I posted it? Does it match the Forum description where I posted it?' and if you're still unsure, PM a Moderator or Admin.