wow, if you can't trust them, I wouldn't get any from them. Not trying to be mean, but make sure you trust who you get them from to know what you are getting.
There are re-homing options as well.
But look around at the photos on here, particularly start a search for photos of joeys or babies, and then do one on adults, you can tell the difference from very young to adult, even close to 1 yr. Mine are about 1 yr, and they look much more mature than the joeys i see in photos on here.
You may also be concerned if they sell them too young.
Try to get as much info as possible, then talk to them again and sound educated, they will be less likely to try to pull one over. Hopefully they are more reputable than that though and are on the up & up. I don't know anything about them.
Just some opinions and suggestions.
Good luck!!!