Interesting one - I know there are auctions for other standing crops (stands of hazel for coppicing, for example, so might be worth talking with your local agricultural auctioneers). Round here I think you would just need to mention it to a few key farmers (usually the retired ones that still go to auctions and meet in the local cafe for a chat every day) that you may have a standing crop you are interested in selling... or you could put up a postcard in the local farmers shop? We chose to get it small baled (paid a local farmer) and sold it off the field as we didnt have storage. That way you take more of the risk, but get more of the money. If you can get someone local to small bale haylage its like gold. We small baled hay and sold for £3.5 - £3.75 as long as they collected on day of baling and it worked really well - word traveled and it all went quickly. Good luck.