The main goal of running an eCommerce website is selling stuff. How you get customers to buy your stuff is another matter altogether. Looking at the websites of major retailers can provide a lot of info as to how this is best achieved. For example, big companies like Amazon and Google have already spent much capital determining what is the best link color, layout, font size, image size etc. to best sell their stuff. Building on such a foundation can go a long way to making your site look just as good as anybody's and you didn't have to spend gazillions of dollars doing it either.
Along those lines, one design area that seems to always be in flux is the ubiquitous 'Add to Cart' button. The glassy rounded Web 2.0 buttons appear to have run their course. Today I see more flat rectangles with simple color schemes being used for eCommerce buttons like 'Add to Cart'. The principle of color contrast of your buttons to the website design still applies, i.e., you don't want your buttons to 'blend in' but instead 'pop' off the page. Being able to find the 'Add to Cart' button should be the easiest thing for people to do, not the hardest. A/B testing should also be part of the process to help determine which style of button is the most successful. For example, was it the one that had the right-facing triangle in the button or the one without? Only way to find out for sure is to test.