This has been discussed before, and I think I remember that Ryan Jordan (and thus BPL) came out on the side of supporting posts providing a better price for the equivalent item. The desire for the community to benefit from the best possible price outweighs the rights of the individual seller to make a little more money, or something along those lines. EDIT: Use the "Search Forums" on "Price Policing" and you'll find his post.
That said, if you are going to post a "better" price in response to a gear swap item, make SURE the item is available for purchase, is exactly the same item, includes tax and shipping, and all that. Also, it is most respectful to simply post the item and price and location in a response, not anything else editorial.
This doesn't happen often it seems, because most BPLers (at least those that have been around for a while) seem to post items that are well-cared-for at very reasonable prices. In any case, the key concept here is respect, something lacking in the communication in this thread above.