The short answer is indirectly, and with a very light touch.
We have all hit sites that seem like nothing but a catalog of stuff being sold. Flashy, blinking banners. Bellowing sound, if you allow it. I don't know how they can sell anything. I run quickly without even a thought of looking back.
When you are selling products produced by others, let the producer do the selling. (If they can't get it done, you have the wrong producer and product.) All you should do on your site is to recommend the product. And to do so indirectly, with little fanfare, may be best. Look at it this way.
When a visitor arrives at your site, the question is always what's in it for me. So you answer this with good useful site content. You provide the information the visitor came to find. Maybe it's air fares to Europe, the cost of renting a house boat for two weeks next summer, all known symptoms of hoof and mouth, or the latest rumors about the price of oil.
Your site has a purpose for being. If all is working well, your visitor arrived in accord with that purpose looking for information you can provide. Do so immediately.
To hit a visitor with a sales pitch is dumb. All surfers know where the Back button lives. And they use it frequently.
One way of looking at your point of view as a webmaster vs that of your visitor is to think in terms of MWR (Most Wanted Response). That is, think of what you most want your visitor to do relative to what your visitor most wants.
It doesn't take a lot of brain power to realize you and your visitor do not ultimately want the same thing. For example, your visitor wants information; you want a sale.
Taking this a step further, it should be obvious that what you want is meaningless to your visitor. Thus you are whipping a dead horse if you do anything other than seek to provide your visitor with exactly what is wanted. That is, your MWR must be for your visitor to find what is needed. To work toward any other objective is to fail.
Suppose your visitor is looking for information about pruning rose bushes. Then her MWR is to find that information. Your MWR at the time of her arrival must be to provide it. If you can, you have accomplished a great deal. You will have drawn her into your site. You have been allowed to demonstrate your resources and expertise. While you may not have made a sufficient impression to assure she will return, she probably will not unless you provided what she wanted.
Now suppose you have a marketing deal with a garden tools wholesaler. That your visitor is interested in how best to prune roses, may mean she is also interested in good pruning shears. Which of the following will bring more sales?
A sentence within the article: "The first step toward good pruning is making sure you have top quality shears. My favorites are made by Diltson. They are simply the best. (Click here for further info.)"
Pop up a secondary browser window and in two inch red block letters toss up: SALE! Save 30% if you act right now.
If you think the second approach is even feasible, you're right in only one sense. It is feasible. Some will jump at a sale. Some will even do so when they have no real need for the item on sale!
But you can not build a loyal customer base with the latter approach. The soft sell in the middle of an article in which you are providing needed information will take you much further in the long run. If your visitor clicks on your link, it will be her choice. Thus at the other end of the link, you will know she arrived by choice.
Enthusiastic support is called for. But so is lightness. And grace and style help as well. Something very simple may work best.
"Being the gardening fanatic that I am, I think I've tried every gardening tool made. Those I haven't tossed, lie rusting in the garage. These days, I've given up looking. Diltson tools always deliver. They work better and last longer than any other tools out there. Nothing beats them.
And my visitors say the same thing. Many thank me for recommending them. [A great place here for a testimonial.] Check it out for yourself. Just click here."
Now look what has happened. If your visitor clicks on this link, she arrives at Diltson's showplace with an open mind, probably hoping to find a better tool. With less than eighty words, you have converted a total stranger into an excellent prospect.
Even if your visitor did not click on either link, you still have a big win here. For one, you have not offended her with a blatant sales pitch. More important, she found what was needed, good information about pruning roses. In this, there is at least the beginning of trust and an appreciation for your expertise. From here, she may explore further or come back later.
While we would like to believe this approach always brings a return visit, it just isn't so. A visitor who does not buy on the first visit, and does not come back, is a potential sale lost forever.
But the more important view is to look at this from the other end. If your visitor does not find what is needed, does not recognize your authority and expertise, there will be no coming back. Period. At bottom, your MWR at the time a visitor arrives is to provide precisely what is needed. It is the only way that offers the chance of a future visit and a further opportunity to make a sale.