I owned a garden center back in South Africa. We would get plenty of people wanting us to sell their products.
A few of them really understood our business and addressed the questions we had (and we normally bought from them). Think about your product from a retailers point of view:
- How much money can they make from selling it?
- How easily does it sell?
- What will you provide to make it easier to sell it (display stuff, advertising, promotional pricing etc)?
- What happens if it doesn't sell (will you take it back)?
- Does it have a limited shelf life (go off, go out of fashion)?
- How big a commitment do you want them to make?
- What aftersales support do you provide?
- Can you meet demand?
- How do we get your product (pick it up, you deliver, by courier,)?
- How easy is it to order from you?
- What payment terms do you offer?
That's enough for the moment. If you are going to use a letter make sure that it addresses some or all of these concerns.
Cheers
Pete