Pottery doesn't change and grow, pottery doesn't die. Buying a bonsai that someone else worked on is not like buying pottery or a painting, or anything else finished.
The main thing is learning how to take care of your trees. Then, if you want to get bigger trees, go back to the nursery and buy bigger TRUNKS. Yes, just trunks.
Ignore the top of the tree entirely. Ignore anything in 1 gallon or less pots. Go to the back where the big trees live.
Look for spreading roots (not twisty, tortured roots... that isn't bonsai. Roots should spread to grip the ground, like wagon wheel spokes) Look for some taper from the ground to about one foot up. Buy that. Then chop it off (in the proper season) and grow a new top and branches.
There's a lot of information on trunk chopping, and nursery shopping. Search this site for the articles and posts. It's an in-between method. For fifty bucks you won't get a really decent bonsai, but you can get an awesome trunk. Then you have something to work on. In the mean time, stick your little guys into the ground and forget them except for watering and feeding. Let them go. You'll be surprised how quickly time passes. There are articles here and at evergreengardenworks.com about how to grow trunks in the ground, apply those methods to your current trees.
Good luck!
Joanie