Hi get them valued first and send them to an auction specialist. Dealer might not give you the best price. You can send your stamps to an auction place in another country that offer free valuation. But contact them first. Here is an examplehttp://www.warwickandwarwick.com/pa...luations.htm
Depending what country you have the most of you may want to find a good auction house in that country. Take some pics of some of the stamps and list them on the forum groups here, maybe there are some valuable ones. You don't want to be ripped off when you can get more money for what you have. If you have later 19th century stamps you really need them valued as there could be an expensive one among them.
Another option is to send them to an on line auction such as http://www.gvstampauctions.com/index.htm
Thereby buyers will see them on line and bid.
But with all auctions you cannot be sure what you will get for them. I recommend the first link to at least get a free valuation. If you send your stamps to them, contact them first, send them registered and well packaged. If there is a valuable stamp they will tell you.
Hope this info is of help