I have an app that has been a consistent performer and has given me stable revenues (in 7 figures annually) for the previous 4 years. I am only a developer and never do sales & marketing, only focus on development. There is a sure shot possibility of increasing revenues by 2x-3x with proper branding & marketing. Either someone buys this app from me or I get a partner with the right skills so that revenues increase 3x. Anything is okay with me.
The two main online marketplaces for apps are Flippa and Fliptopia. Apps usually sell for 10-20 times the last month's revenue. Listings selling for over $50K are few and far between; must close at $500-$5000. For assets worth considerably more, you would need to find investors or a company where there would be synergy.
For anyone needing help in this area, feel free to reach out to me.