Sorry my bad english. But I'll try to explain something:
The ONLY criteria for printable models is CORRECT geometry of the model: closed solid mesh without intersecting and overlapping triangles that complains with the printer (or technology) specifications (minimal wall thickness and good printable details of the surface). Placing (orienting) models on the printers table is the job not for modeler but for people who PRINTS those models on their own 3d-printer.
Placing supports is also customers task. The modeler/designer can't know what printing or CNC equipment and techniques will be used by customers: e.g. CNC mill, Solidscape printers, SLA, MSLA etc... So doing USELESS job like preliminary model positioning can take too much time and give lots of unneeded results that will never be used by customer in the real life.
Also exists option to contact seller/modeler/designer and discuss with him/her all questions you have. I think most of fair cgtrader-people is open enough for communication with customers/buyers.
If you can't to rotate/translate the model and place supports like you want and need or if you don't see or don't "feel" the models "printability", just don't print anything. Or make model yourself to get PROPER (in your vision and opinion) model.