Depending on which type of business you own, either online or a walk in shop, you may want a few different types of mannequins. You can find different ones are made of fiber glass, plastic or metal, even wood and styrofoam.
For online sales your best bet would be the lightweight plastics. They are affordable, and sturdy enough to use at home or wherever you want to take pictures of your piece to prepare for selling.
For your physical store location on a ground display you would want sturdier fiber glass, heavy plastic mannequins or wooden mannequins in full body life size. You can expect to have customers that walk through your store and enjoy the touch and feel of the merchandise while browsing. Mannequins which stand their ground are best for these times.
Many shoppers also bring young kids along which like to play wherever they're at. If this is the case you will be glad you have mannequins that don't tip over easily. Damage to your clothes and the mannequin will cost you money in the long run, not to mention the clientele you will lose if you ask them to pay for damages.
If you are displaying just a shirt, blouse, or top high above on a wall or from the ceiling, a half size, upper body torso mannequin would be your best choice.