Short and to the point, you have companies that specialize in buying apps, and marketplaces where you can auction your app.
Each has its Advantages and disadvantages.

Big app buyer:
https://www.kingfiloholdings.com/

Marketplace:
https://flippa.com/

There are a couple big companies you can approach to sell your app:
https://www.kingfiloholdings.com/
https://empireflippers.com/
https://flippa.com/

There are a couple marketplaces and VC's that specialize in partnerships, and buying/selling apps.
Firstly I suggest to get an free appraisal:
https://www.kingfiloholdings.com/app-appraisal

Than try the top app marketplaces:
https://flippa.com/
https://empireflippers.com/