Retire Early Assignment

Hello,

1. Can I assume there exists an infinite amount of stock from where the users can buy? And also an infinite amount of cash so users can sell stock immediately? So everything happens in a virtual economy.
2. Or I should assume there is outside system (stock market) that does the real transactions and I have to address all the communication with this system?

I can not decide what is right to assume. First assumption seems to make the system too simple and the 2nd assumption seems to make the system too complex.
Is it even right to assume the existence of an additional external system?

Thank you.

I have the same assignment.
I think it is OK to have an assumption as long as it does not make the system too simple.

Which ever route you go, be sure to document the assumptions you have made.