Don't respond to the text that you recieve from them. Sometimes they want to send a message to someone else but by mistake sent to you. You should find address of that company and do talk with them if you never rented a car.

Yes, you can rent a car if your age is above 20. You should make your driving license .

I would say you can find many sites to buy and sell cars online but you have to find with the help of deep research.