Is there a web site like kelly's blue book for trailers? How can I judge the trade in or resale value of my 1999 Trailet 2 h straight load BP with dressing room?

Hello SN .. There isn't a useful "blue book" for horse trailers. There are too many different trailers and configuration to be useful. Plus the local market and trailer's condition figure greatly into the value.
I would sugest you look in your area for a trailer like yours in similar condition, and price according to the speed of sale desired.

A low price, sweet enough to draw flies will surely draw a buyer too.

What an interesting way to get people interested in reading! Book trailers are like movie trailers, but for books! You can find them all over the internet now, but here is a site that's featuring them on YouTube. http://www.youtube.com/booktrailers