REAL PEOPLE
Will they meet you? Real people will have no problem meeting you and establishing their integrity. At least have a conversation on a real phone ...not a video call from an unknown location.
Will they TALK to you or only communicate via e mail or text? Oddball e mail addresses are also a sign of irreputable outfits.
Can you ask around to verify their integrity by getting referrals?
None of us are perfect but we can often recognize poor grammar that is indicative of a foreigner unfamiliar with your local language.
REAL PRODUCT AT A REAL LOCATION
Can you go to their facility and see their stock? Do they have a physical location? Are the photos you are seeing of their products, photos they have taken? Or stock photos downloaded from the internet?
Does the seller have a physical location you can go and see the container...look inside, walk around and choose your container? Often these scammers have an address on their ads that when googled shows a ramshackle house or a vacant lot.
PRICE
If the price seems too good to be true....it is! It is a competitive market and highly volatile in this current economy. The prices may fluctuate somewhat between dealers but not thousands of dollars.
Do they encourage you to shop around and give you advice to get the best deal for your needs? Advise on transportation, set up etc.
Some places have great deals on containers, then surprise you with the delivery fee! Others may deliver it to your location and drop it in your driveway, leaving you to figure out how to get it to the spot it needs to be in.
Make sure all aspects of purchase and delivery are understood.
VALUE ADDED SERVICES
Is the seller able to answer your questions? Proper placement, adequate ventilation, local by laws?
Finally...do your homework, ask questions!