I would set an end date of the offer period, so 
people will know a specific time of when the time 
to offer closes. I would also specifically say 
what is for direct sale, and what is up for 
offers. I also believe that the offer amounts 
should be kept private, and not on the sale page. 
You should let the buyers know ahead of time if 
this will be a one-time offer type of deal, or 
more like a silent auction.