About to buy an f5s box - is there anything I should be aware of?

Hi all, as the title suggests I'm about to buy a skybox f5s. They seem to be available from a verity of sellers on ebay and amazon, will the boxes all be legit?

Also some sellers say that the 'latest' channel list and firmware comes pre-installed, will I be better using AJ's channel list?

Cheers

T