Fabric for collar covers always came from the $1 or $1 a yard bins in MY book. I've made some, and they typically don't sell well.
I think you'd end up selling at a stall at a flea market or craft show, and although you'd end up making a few bucks, it's not something you can make a living on.
In the end, get as wide a range of items to sell, make homemade dog biscuits, do collar covers, dog beds stuffed with cotton and a bit of cedar shavings, cat toys, knitted or crocheted items, anything that is pet related. You'd need to have a grand array to catch people's interest, and they might pick up a few things.
I certainly wouldn't use that site to buy any materials from. Everything you need is available locally(for the most part).