There are several ways you could do this, but how feasible they are depends on your technical skills and also the skill of your customers:
1) iframe - this is probably the easiest, create an HTML page they can load into their website
2) create an XML feed, something like an RSS feed containing your products. They can then load this and integrate the products in their own site using PHP or some other server side language
The last two will require more work, but will look more... seamless on the customers site. The first option is the easiest, but iframes always look a bit disjointed to me.