For merchant specific rebates, manufacturers sometimes require the merchants to provide them the sale information that qualifies the rebate. If Beach Camera does not send your information to Canon since your purchase is from eBay rather than directly from BeachCamera.com, then you are screwed.
Think about it. If Canon does not care whether to honor rebates for marketplace purchases, why the heck they made up such terms and conditions on the rebate form. Of course you can still take a chance, but you are jeopardizing your $400. Good luck!
I'm being a bit paranoid here, but the rebate form states that:
As far as I remember from my 50D purchase they send you a "normal" receipt that doesn't mention EBay. Does that sound accurate? Thank you!