on my site, I would like normal users to be distinct from business users (from companies), consequently I would like the functions of the osclass pay plugin to be exclusively for "company" users and not for normal users (to whom on my site I would like it was only allowed to post 10 ads for free and not to use the payment functions) is it possible to implement such a thing?

@Nightnoir
Take a look on Business profile plugin
https://osclasspoint.com/osclass-plugins/design-and-appearance/business-profile-osclass-plugin-i89

I already have the business profile, but I wanted to know if the osclass pay plugin can be limited only to those accounts excluding normal users?

Just with custom modifications. It does not make too much sense to limit own monetization options 

yes I know, but my site is mainly focused on shops, and for normal users it just leaves the possibility to sell a few products

@Nightnoir
Well easiest way will be to create own function to check if user ID is business or not (based on business profile, or based on company field) and condition everything you do not want to show to normal users using this function.