To prove to someone that you sent the coins to them, the easiest way would be with a video screen capture.
You can download 'Freez Screen Video Capture' for free at this web site
Record a short video of your wallet and you sending the coins to them. There is no way anyone can access your coins by showing them your wallet addresses, so it is perfectly safe to do this.
Upload your video on a free upload site somewhere, like Megaupload
, but make sure the resolution on the video is untouched and you can still easily read what is going on. Click here
to see an example of undeniable proof of sending a bitcoin.
Also, the feedback system works great for seeing who is trustworthy. The posts users make are non-modifiable, and when a thread link is posted in the feedback it will stay there for all to see.