They want to hire someone who has the ability to trade large amounts of bitcoins. and they have this term to be done as a proof.
~snip~
Think about how meaningless that request is, because the fact that you will receive coins from a wallet that has a minimum of 500 BTC does not prove your ability to, as you say,
"ability to trade large amounts of bitcoins". That would only prove that you are capable of finding someone who has that number of BTC, and to me it all seems like someone trying to identify owners who have 500 BTC or more.
It doesn't seem like any honest business to me, and in any case, the request is strange to say the least.