I would have a look on ebay first.
If you do want the auction route, you could try Adams or Whytes or even go for the bigger guys, Christies or Sotheby's.
The big auction houses in Dublin mainly seem to deal with antiques, fine art, furniture, etc but their counterparts in London certainly do have speciality auctions for almost anything you can think of. Auctioneers might charge you for a valuation so that is something to bear in mind in relation to any potential sale value. Also, you will have sellers' fees to pay the auction house if any of your items are sold, and possibly, though I'm not sure, even if they aren't.