Unfortunately it's impossible to make it such that the system will allow a WTB instead of a FS thread before you reach the 25 post limit. Whining about it won't get you anywhere either.

I needed an engine before I had my 25 posts but I just sucked it up and within a few days I had more than 25 SOLID posts under my belt. It's a system that helps us weed out a large portion of the degenerate fucktards that are out there just trying to make a quick buck or scam us out of our stuff.