OMG, this is the best laugh I've had in a long while at this site! I heartily agree with those that efforts at selling rat meat or rat fur at nearly any market would be met with derision and disgust at best.
To be constructive, however, IMC there's plenty of wild game around that even low level PCs ought to be able to easily hunt in the wilderness surrounding most towns and villages, if they are smart about it, and if they are really that poor.
I also agree with the notion that players attempting to do this brings up a great opportunity for role-playing in the campaign--one that could really bring the local town and market to life quite vividly.
On the topic of guinea pigs being a delicacy in South America, we here in the USA don't need to look even that far. Nutrea, a kind of swamp rat, is raised for eating in Louisiana and elsewhere here in the South. Apparently, it is pretty tasty and nutritious, though closer to pigs than the Dire Rat. I would hasten to point out that many in the fine state of Louisiana are Cajuns, folks with French ancestry, and one thing the French are known for is a rather wide palette. They eat snails, which most people find disgusting, but I gotta say for myself, escargot simmered in butter or olive oil with garlic is one of the best things I've ever eaten. And let's be honest here--shrimp and crawfish, quite delicious to eat, are nothing more than insects that live in water. Same with lobster and crab.