What do I need to know before I make an offer to a Seller?

At the very least you will need to know:

1) What is the pain?. Why do they want to sell.
2) What price do they want. Would they prefer a higher price or higher monthly payment?
3) What are the payments for mortgage, taxes, insurance, and the balances on all outstanding amounts.
4) Is it rented? for how much?
5) What do other similar houses sell for
6) what do other similar houses rent for.
7) Etc.