Believe it or not but indoor bonsai are quite different from outdoor bonsai.
The main reason being that they are from completely different climates and are used to very different conditions. They key is to get to know the new species before one makes their new purchase.
There are some key areas one should focus on.
Leaves are very important since these are the solar powered power houses of the plant. These should look healthy. It's good to be familiar with what the species of tree you are intending to buy as a bonsai and looks when growing normally. This will help you to know if the leaves look normal even if they are slightly miniaturized.
The leaf colour should be normal for the season. If it is the peak growing season for this type of tree then the leave should be the correct shade of green. If not it can indicate poor health or nutrient or other deficiencies.
The leaves should not show signs of damage. Damage can be from pests such as aphids, worms, slugs, mites, weevils and other bugs. You don't want to take these critters or their eggs home and infest other specimens you may have.
Damage to leaves that is not from bugs etc. can indicate diseases. These diseases can be things like mildew, molds and even viruses. Once again you don't want to invite any of these home.
Its also not wise to try to be a plant rescuer unless you really know what you are doing. Firstly, it can expose your other trees to whatever is ailing the one to be rescued. Secondly, one ends up financially supporting the negligent grower or nursery and leaves them no incentive to rectify their practices.
It's like buying wildlife or pets in order to rescue them. The moment money changes hands the incentive to continue the cruel business model continues.