Use your intelligence when it come to buying off people. However if you cant rely on it - DONT do it.

A random person coming on a site leaving an email address advertising MJ with one post (although posts are not always a tell tale sign) with no one backing him up is likely a scammer. But to confirm a scammer usually will ask for money prior to delivery, thats why CAD (cash after dilevery) people often are reliable.

When it comes to websites you have to be fucking stupid to get conned. Just by typing in the websites name into google you will probably get a couple of reports on it.
Also by looking at the site itself you can tell sometimes. For example High Grade Seeds site. www.highgrade-seeds.com it says at the top "most potent" with no scientific evidence to back it up.

Also by having a look at this
http://boards.cannabis.com/showthread.php?t=8686
A tactic many scammer use is "posing" using fake accounts to look as though people have ordered off the site notice how a guy names Kypp Cox with only 14 posts says this

"I personally have bought several hundred dollars worth of seeds from HIGHGRADE SEEDS and my experience with them has been nothing but spectacular!! I have bought seeds from several places and none of them has been as nice, helpful, and respectful as HIGHGRADE SEEDS. I recieved my seeds within a week and the packaging is the best I have ever had. They always have given me extra seeds. As a matter of fact they have given me alot of extra seeds!!! They always answer my E-Mails within a couple of hours and are very nice. I believe HIGHGRADE SEEDS is the best SEEDBANK there is available today."

Bit suspect isnt it? Also when looking in the contact info on a site (all good sites should have this, if they don't they are scammer) On HGS site (as previously pointed out by Torog prior to HGS taking it down) they had cannadia.net's phone number on it. What kind of a site uses other people's phone number?!

Prior to ordering from ANY site you should always email them about the product, bear in mind these people are trying to sell a product to you and they are basically a "waiter". If they don't answer emails prompty (or at all) they arnt reliable either.