how do u write a letter for a business and persuade them to buy my comany?
No letter can do that. Your company has to be worth buying. If it has the right customer base, expertise in the right products or services, an appropriate debt/asset ratio, and a good reputation, it will be appealing to the right buyer without a letter.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.