Do not Bump your own thread.
Here's an example. You create a thread on 7/18 @ 2000. You can't post again on there till 7/19 @ 2001 (this allows for the 24 hours to have passed). Time is kept at the bottom of the forum (and its just below a private message). Every post you make is time stamped. If 24 hours have not passed but you want to add cards to the for sale/trade thread, you need to edit the original post.
The image below is a bad example as its only a 1 minute bump...but 1 minute, 20 minutes, 2 hours, 23 hours 59 minutes....its a bump - help us avoid sending the dreaded Thread Bump Warning PM...or worse yet Suspensions!
Do not try and circumvent the rules by posting cards in duplicate areas. This is typically treated as bumping and will not only the thread bumping infraction, but both threads will be deleted!