Make sure it's completely built and ready to go before you pay anything out. I got scammed out of almost 2 btc by someone offering to build a rotator to my specs. Ignore all the hard luck stories, and use escrow, too!
Thanks OmegaCollector for the heads up 
, yes I will only use escrow for this matter in particular as it's a sensitive one and could easily be scammed ... hope I could find a decent person though