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/