Condition and Serial Numbers

[Allen Michaels-migration]

I am new to the hobby and seek reliable answers to a few questions and apologize if my greenhorns show.

 

In US Federal Reserve Notes, what would warrant 'worth more than face value' when considering only the serial number as being a near solid, near binary, near radar etc? A dealer had told me 5 digits or more would carry a premium but only if it were in an uncirculated condition.

Is a Federal Reserve Note whose serial number is collectible but has pen marks, foriegn stamps or the like worth collecting or has it lost its value? What are those peculiar stamps? I have a solid 7s one dollar bill but it has a tiny red elephant. Can it be somehow removed and retain legitimacy?

 

Thank you in advance for your answer.