How to Get Multiple Product Sales as a Seller
Selling a product is perhaps the oldest and most straightforward business in the world. This is a simple exchange where you give someone an item, and they give you cash in exchange. To make this work you need to ensure that they consider that item more valuable than the money you're charging for it, while at the same time ensuring that you spend less than that amount of money in developing it. 
Those are the basics, but when it comes to the difference between a successful product business and an unsuccessful one things get a lot more complicated. This is a very fine art and precise science where even slightly tweaking a single variable can make all the difference.  
Some of this is to do with marketing of course, but more important is what you do once you've got the sale. Once you've sold something to someone once, how do you ensure that they want to come back for more? How do you capitalise on that opportunity and go from a single sale to many? 
High Quality Products
The most important factor here is the product and this is why it's so important to invest money in researching and developing/acquiring the very best products you can. Many companies here will think too much in the short term and aim to simply sell a single item. What they aren't recognising is how much damage a bad product does to their reputation and their brand. Investing in quality products means investing in your customers and going from one off sales to lifetime loyalty. 
More to Buy
If your goal is repeat buyers, then you also need to sell a product that benefits from/requires multiple purchase. If you sell a TV, the soonest you can expect to get another customer is in a couple of years. That's why some companies will cleverly create complementary products that allow them to take advantage of their customers' newfound familiarity with their brand. 
This is also the benefit of having a brand in general. That way, you can sell multiple products but leverage the good will built by one in order to sell many more! Stick within a single product category, but create variation and encourage people to “double dip.”
Rewards
Finally, if you want to go one step further, why not entice your customers to come back by offering them some kind of incentive? If repeat customers get money off, then that will give them much more reason to come back to you instead of going elsewhere. Combined with the points above this is really just the icing on the cake. 
