Re: metabones & canon 400/5.6 purchase question
I know nothing about the Metabones adapter, or even the Canon lens, apart from what I've learnt from Daniel's comments.
OTOH, I've been trading on eBay for several years now and they have a very good buyer protection program.
Also, I have read somewhere that since the popularity of m4/3, and the proliferation of lens adapters for a huge range of brands, that used lens prices have gone 2X even 3X that of a few years ago. That's the asking price from "dealers". Actual auction prices don't seem to be near as high.
I don't have any specific numbers to quote on that.
So, don't be afraid to buy eBay, just have a good look at the prices.
Twin E-620/70-300mm/14-42mm/pinhole lens, plus 8mm FE, Tokina AT-X 90mm macro, and Zeiss 500/8 mirror.