Easy way to churn Profit

[Covoxer]

Not sure if it can be implemented but how about the ability of being able to buy any plane once if you have the finances, then
cannot buy that plane again until you restart the simulator?

I would fix it this way.
Keep the list of sold propositions - in simple variant it should be only the number of proposition for every sold plane.
When user buys an aircraft, FsP adds used proposition number to the list.
Next time when user looks for propositions, those that are on the list are simply hidden.
When you make a flight, the list is simply erased.
That's it.
I'm sure that Dan can fix it this or any other way.
But it is not really necessary.
You see, if you want to stay realistic, you will never use this issue anyway, and if you want to cheat, you'll find a lot of other ways to do this.
So....