a good reputation is the 1 thing a look for while shopping online. if i dont know the store personally i will ask about on sites like toms, hexus net and overclockers for user opinions. its how i first came to use ocuk and ebuyer. i dont buy of ebay no matter how reputable the sellers are. not because of bad experiences i just dont like it.
as for email scams. the majority of the time there know scams so googling the title or the email addi it came from will tell you its fake.
always follow the basics of if it sounds to good to be true then it probably is.
1 more tip. if you get an email from your credit provider never answer it directly. always go to there site and log it. the chances are they will have tried to contact you through that account before they emailed you (if they even did). if theres no messages there then contact them directly via there site. paypal and others will happily tell you if they sent the emails and will happily take the details of any suspicious emails you may have received under there name. if they get grumpy then remove your funds and close the account...
Use WOT and webputation (addons) in firefox to give you ratings on websites.
When you recive an email with a link, if you go over it, you can see what website it's relate to (in good email programs) So you know if it's a scam or not