Well I see few ways.
1) My suggestion of getting rid of 'broke and can't succeed anymore' parties. Leaving them in the player history as failures.
2) Let them borrow equipment and pay back later with a bit of premium. This will be risky to shopkeepers.
3) Let shopkeepers invest in such a party but in return take everything that party earns for a certain number of missions. If the party they invested in fails, take all their stuff back maybe?
