Hi,
we have developed a plugin to mimic the ebay make an offer feature for virtuemart,
it allows you to
set minimum price on each product
restrict customer's try time on each product in several hours
reject offer while below min price automatically
accept offer while above min price automatically
add the product with the accepted price and checkout as normal
offer price shown in invoice and order
Product home page HERE (http://virtuemartoz.com/other-virtuemart-extensions/make-an-offer-best-offer-for-virtuemart.html)
Demo Here (http://demo.virtuemartoz.com/index.php/shop/pagination/ghd-6-32-detail)