When I ordered one of the eBay deals through BeachCamera a few months ago, there was a "regular" receipt in the box. Not sure if they'll do the same, but just FYI. So assuming they haven't changed their procedures, you should be good to go for the rebate.
Get the $400, sell the printer, and you have a very sweet deal on a 5K mkII
The Mark 3 I got from them clearly says ebay on them though. Maybe they do a special receipt for these rebates?