Can the customer buy the apps on App Exchange platform without subscribing to sales force?
The following thread has the necessary answer that would help you understand the differences between Salesforce's ISV and OEM Appexchange Partnership Models.
Also, watch this video to understand more about the Partnership models - https://www.youtube.com/watch?v=_oV9LthQ5O0
Yes, you can build OEM apps that customers pay you for and then you pay Salesforce for the licensing. Technically, you can market something on the AppExchange that has nothing to do with Salesforce, but my guess is your listing will get taken down.