I have just had someone pull out of buying my caravan - won't bore you with the details, but they were the second bunch of gits to screw me around.
I have always found the following couple of methods of revenge both satisfying and entertaining.
1. Collect several nice & fresh dog turds, place them in a reasonably thin paper bag. Then place the paper bag (full of turds) on the offending persons doorstep. Set fire to the paper bag, ring the doorbell and retire to a safe distance. Most peoples response to seeing a bag on fire on the doorstep is to do all they can to stamp out the fire. Imagine how satisfying the resulting scene is!!
2. On a single sheet of paper simply print these few words; THE POLICE ARE WATCHING YOU - ACT NORMAL. Send this by post to the object of revenge ensuring there are no clues as to who posted it. Probably best to post it from a different postal area! It's surprising how suspicious people look for a while after receiving this little note. Tee hee.
Cheers,
Dave.