Nice work. I am a 3ds Max Modeller only, so I personally sell my items with Max-vRay as native format, with Max-Sanline, exported OBJ, 3DS & FBX formats also.
I find this is a wide enough variety for mainstream users & allows end users to further convert for other formats & game engine formats too.
I have had a number of special requests for Maya native file formats to be included too. Apparently certain data is retained easier with native Maya instead of importing from exported formats.