Never buy out of an alley or from the trunk of someone's car. There's no return policy, and you're much more likely to get a cheap fake bag rather than a great replica.
Check how long has the site been in business and ask for real customer references.
Only buy from sites that offer reliable shipping with tracking and accept paypal or a credit card for your first order.
Make sure all items come with correct accessories, dustbag, paperwork (care cards and/or care booklet) and markings including serial numbers and date codes.