The BB's classifieds top out at $1000 in value for the items being advertised, so it's not likely a truck or trailer would fall under that line! If they did, you could post it in the "miscellaneous" section. ($1000 ads cost $20 to run; $250 max ads are $10.)
Otherwise, check out the Chronicle's regular classifieds, which are featured on www.chronofhorse.com--with or without an accompanying print ad in the magazine. It's only $25 for 2 weeks online only!