Should not hurt at all to haggle a little. I would ask them if they could get it to you at $425 OTD--perhaps a little higher depending on your sales tax rate. Personally, unless its pristine, I would really have to be jonesin' for a 5903 specifically to pay 450.00 with all the nice 5906's out there going for ~300.00 but that's just me.
Make sure to field strip the gun and look closely for signs of wear on the barrel, guide rod and slide/frame rails.
If you do get the gun, make sure to post pics