Should people be allowed to sell organs for financial gain?:
NO !!

Firstly, because this essentially puts a price on human life and dignity. Life can be given, it should not be sold.

Second, because it's usually a short term decision, that can be dearly regretted if you live long enough. You will of course sell only organs you can live without, a kidney or an eye... but what if, later in life, your remaining one fails, and you don't have the money to replace it? You're in the doo doo, possibly dead if it was a kidney.