I have two suggestions to help. First, is to contact firstname.lastname@example.org for members of the International Bond and Share Society. If you are not a member, it gives you another reason to join and support Scripophily http://www.scripophily.org/. The auctions are well run using items consigned from other members. If you decide to participate please remember that the Society does not perform valuations and a brief introductory email contact is the best first step. I recently purchased an item in the June 2011 auction and although I struggled with the currency, the Auctioneer was very understanding, thanks again. I am looking forward to the September 2011 auction.
Second, if the certificates have some value, contact your favorite dealer and see if they would be interested in either an outright purchase or a trade so you benefit as much as the dealer. What is important to remember is that the dealer still needs to cover expenses and can only offer a percentage of what it can attract in a full retail environment. Still, that amount can still be above your acquisition price. I recently traded certificates that I had multiple copies for two that I had been watching for some time. At the retail level, once the dealer sells the certificates I traded, they will yield a higher price than what I received, but the operative word is "when" they sell. The dealer must catalog, advertise and store them and if they know a collector interested in a certain subject, it is because they spent considerable time building personal networks of collectors providing insight and expertise.
On a separate note, this month I recently acquired a nice certificate of The Barnes Automatic Car Coupler Co. with an amazing vignette. See the Barnes certificate below.