Journal: Legal snapshots of web pages?

I recently purchased an exercise bike online. Long story short, it didn't match the description on the seller's website, and when I asked about it, they altered the page (the original version is still in Google's cache). They also sent me an e-mail saying:

"Thanks again for your purchase. What are you finding to be different about the bike than the listing?"

With something as ephemeral as the web, how does one PROVE that a page said something at one point in time? Is there a way to make snapshots of pages for legal purposes? Is there an online service that does this? How legal is a simple printout?