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Thanks in advance!
If your keyword is "bunny food" or whatever, make your landing page about bunny food. Now general keywords are a bit trickier, because you don't know what the customer was thinking when he or she clicked on "pet supplies," but you can still work with what you have. Give people enough choices to ensure that one of them is right but few enough that there's no confusion about the action. This might mean that you have a bunch of different landing pages, sometimes you can get away with only one.
As for the format of the page, that's really up to you, and it depends on the product offering. Some products do well with a sales letter, maybe 6 or 7 pages long with a couple pictures and testimonials to break up the design. Some products really move with a few bullets and a big picture.
Take a look at how your merchants are selling their products, they know their product better than anyone (one would hope!).