You may or may not realize it but there is a good chance that those "stores" already have an API for sharing pricing data. A good example of this would be EBay. I posted some things for sale there and I noticed when I searched for similar items I ended up getting several web sites that were displaying my item for sale!
Chances are that someone is just selling a kit of some sort that gets the information for them. That being said it can be difficult to write your own even using their API. EBay has modules on CPAN for doing this stuff, I looked at it and it is written completely in EBay techno babble. (God forbid they write something simple and comprehensible to the average joe).
Basically, I am suggesting that you might be better off trying those published API's instead of rolling your own. You can very easily encounter a lack of conformity for the information that you want to get, as well as invalid HTML. That is not even touching JS issues.