Question re: Affiliate Marketing for Online Stores

Hiya

I'm fairly new here....more of a lurker sapping up information than anything else so apologies if this is a common (annoying?) question. From what I've read, products - physical or virtual ( be they ebooks, an SEO tool, a cure to acne or whatever) that are marketed directly through a sales letter convert at a much higher rate than those sold in a broad online shop.

So, my question is does anyone have any tips for maximising the conversion of products that are sold through an online store? Actually, i should clarify - I mean as an affiliate marketing such a product, what can I do on my site that will help convince the customer to buy once they reach the online store (separate to my site). And beyond that, does anyone here earn a decent amount from being an affiliate to such online stores?

I'll give an example. I'm a rugby fan and I've noticed a particular jersey gets searched for a lot on google, but does not have a lot of competition. If i set up a site focusing on this particular jersey, and then linked to the online site of my choice, would the conversion rate be so low as to be not worth the effort?

thanks, i appreciate all answers. Dave