Auction properties with no price

I am trying out this module but I sell a lot of properties by auction so I need to be able to either leave the price field blank or write the word auction. I can't currently do this. Can this field be made not required or allow alpha characters or perhaps even create a category of listing for "Auction" that does not require a price but might include auction date. A category for rental might also be worthwhile that asks a series of questions related to rentals.

Thanks