Share your wisdom with me please

Hello there everyone. I am new to this forum and have a few quick questions that i hope some of you experienced people could shed some light on.

I am thinking of starting a small side line business buying second hand laptops and selling them for parts. Please answer the following questions for me.

1) How do i price a laptop to gauge my return of investment?

2) Is there a way i can test each part seperately to check they are in working condition?

3) How do i find out what models of laptop my parts will fit into? Will different parts fit multiple models of laptop?

4) How will i determine the demand for laptop parts from a particular model?

5) How can i check how old the laptop is online before i buy it?

I know these may seem like vague questions but any wisdom would be greatly appreciated.

Thank you for reading this and thank you in advance for any replies.

Joseph