I was going to mention values falling but I didn't want my post to be too long.
Yes, there is a risk that between the time of consignment and sale card values will fall so much that you will not be able to cover the advance. Also, I was not saying the advance would necessarily be used to cover other purchases...it could be used to cover the cost of a new hot water boiler.
I guess I am just saying that if you sell $6K worth of cards you actually have $6k cash as a result and that an advance just means you are are getting $2k now and $4k later.
There are of course other variables involved and in theory the prudent thing is to not get an advance and wait until the auction is complete and you have received your check before spending (or saving) the proceeds.