Two Ideas:
1) Run some paid advertising to get you in front of potential buyers. Who would be the ideal customer (in this case 'ideal buyer') for this asset?
The benefit of a marketplace is that you find people who are ALREADY looking to buy. That's nice. But many times people aren't aware of the benefits of software. Map out the top 3-10 benefits that the buyer would have. Then run ads which get you in front of them. Sell them on the fact that this application is the answer to their problems. Rather than going through the pain of developing it themselves - they buy from you.
2) Established Marketplaces. Of course you mentioned Flippa. Flippa may not be the best placed for a seller because it's a buyer's market. Also 90+% of buyers there want sites, apps, etc with demonstrated revenue.
Here are some other marketplaces though you may find helpful:
Marketplace.Sitepoint.com
Websitebroker.com
Buysellwebsite.com
Aappbusinessbrokers.com
Apptopia.com (for mobile apps - may be able to discover some buyers interested in Windows appications).
Good Luck