Format of models

For sure, more offered formats will increase number of potential buyers. Is it worth it? It's up to you to decide. If you already have those extra programs, have good knowledge how to work with them and it doesn't take much time and effort for you to convert the models, then why not, go ahead and make conversion. After some time you will be able to tell by yourself if that will increase actual sales. But in my opinion, you are already offering number of formats that covers 80-90% of buyers' needs.

I guess it depends. I normally only provide fbx or obj as my models are all designed for game engine use. Sure, people use them for others, and yes I would get more buyers if I also uploaded other formats, but I want to stick to what I do, and that also means the community of people that follow me or know me, know I do said game models.