CGtrader made a infograph a while ago with a lot of interesting information regarding different types of models and file formats.
https://img-new.cgtrader.com/uploads/blog/04db7daa-a47d-4188-bfdc-ae17e3ce0a89.png
Unfortunately, skp files did not even make the list in top 10 used file formats (and the number 10 is only good for 1% of downloads).
That said, that does not need to be a problem. It is important to just make sure you also export to other file formats. I myself use blender as my modeling software, and blend files only make up 1% of the files sold on cg trader. Even so, i have quite a few sales because i also export to OBJ (good for 18% of downloads) STL (good for 12% of downloads) and a few other formats.
only a few of my costumers are actually interested in the blend files themselves. they often leave them untouched.
So what ever you do, also export your files to different formats, especially OBJ. If you don't, you are cheating yourself.