Sometimes the merchant will provide professional looking display ads in standard IAB (Interactive Advertising Bureau) sizes. If they look good, save them all to your docket folder so you don't have to hunt for them later. Be sure to download any other artwork, logos or photos provided as well.
These ads can be useful for doing media buys, retargeting, or putting on your websites. One of my favorite tactics is to write about a product on a landing page, then tell the prospect to click on the ad in the sidebar.
The downside is that the ads may, or may not, have been tested, so you have no way of knowing if they'll attract clicks. The other downside is that your display ads will look like everyone else.
If you want to make your own ads, check the fine print on the offer page. Sometimes the merchant is ok with it. Other times you have to use the ones provided, or get any ads you create approved by the merchant.
Having a graphic design and advertising background, I always want to make my own ads. But if I want to start retargeting right away, I'll use their ads just to get the campaign running, then replace their display ads with my own later on.