It depends on what the artist/modeller says is acceptable usage. Some say to use the model however the downloader wants to. Others say it can be used in whatever manner, but can not be distributed in any way, including posting on another web site or e-mailing. Some only allow usage in non-commercial projects. If someone takes a model they downloaded for free and tries to sell that model without the artist/modeller's agreement - that is flat out wrong! If they post it for free on their own web site or use it commercially (such as in a picture which they are paid to produce), then the legality/ethics depends on what the usage agreement on the website and/or text file said. If you're not sure, you should ask the creator or assume the strictest definition possible - if you stand to make any money off the model, don't use it.