The AI in this game can't really tell the difference between a mortgaged property, and an un-mortgaged property. This means an AI is just as likely to trade $120 for Connecticut Avenue mortgaged as un-mortgaged. This can be used when playing against 1 AI, by collecting as many properties you need for the AI to accept the trade, mortgaging them all, and trading them for all their money, and their properties if needed. Assuming the AI doesn't have a high enough net worth to pay off the fee to keep the mortgage on the property, they'll go bankrupt, and you'll win the game. Besides the quickest possible Monopoly game (seen here https://www.npr.org/sections/thetwo-way/2010/06/how_to_win_monopoly_in_21_seco.html ) this is the quickest known strategy that is easily replicable.

Edit: Because of a recent patch, it is no longer possible to bankrupt an AI with a trade, making this guide outdated. Also, it seems that AI can now tell the difference between a mortgaged property, and an un-mortgaged property. However, it still is worth mortgaging properties and trading them away for half their value if you have no use for them.