When you purchase used auto parts from online salvage yards, make sure you have a clear description of what you're buying. For example, a door frame may be just that, the frame, without any of the other components of the door, which you'll have to locate somewhere else. The frame may be cheaper than a full door, but in the long run, you'll pay more and have to assemble it yourself, which isn't easy.
In addition, some parts could have damage you cannot see in a picture, so you should be able to return the part at no cost if it doesn't live up to your expectations. Look for reputable online salvage yards that offer guarantees, and don't purchase auto parts from just anyone online. Go to reputable online junkyards for the best products and service.
