I agree with the answers above. We cannot trust everyone even in real life, so what about in internet?!
As I don't have business by internet things I can trust but with limits. I have a cousin who programmed a software that costs 3000 $ for a Swedish sociaty, but they didn't pay him until now!
And also people (usually hacker) send you sometimes malicious programs (Trojans, viruses,) to get the access to your computer or make a mess.
The tricky scam is:
They send you a files with an icon of an image ( jpg ,gif ,bmp ,...) ,and put lot of spaces and only in the end the extension , the file name will be like this :

myphoto.jpg ( here only space ) .exe

usually people execute directly the file when they receive it directly, and Puff the Pandora's Box is open.

Peace Out,