What do you do when you have a collection you no longer want or you inherited a collection and decide stamp collecting is not for you. It's OK. Stamp collecting is not for everyone.
You have a few options and I will discuss them below. Hopefully you or the owner of the collection kept good records of its inventory. Knowing what you have makes it easier to value when you decide to sell them.
If you don't have an accurate inventory it's wise to take one to the best of your ability. Some stamps are easier to identify than others (duck stamps are very easy) so do your best and the odds of you having that $50,000 stamp vs the $0.40 stamp is unlikely.
I suggest reading Top Dollar Paid! by Stephen R. Datz. I have a few different editions and things really haven't changed from my earliest edition (1989) to my latest (1996). Great information on how to sell your collection.
For large collections try to get them appraised (that will likely incur some cost). Most of the value of large collections resides in a small percentage of the stamps so those are the ones to identify and have certified (1) if possible. Once you have a working idea of what you have here are your options.
1: Auction. If you have a nice collection with a few higher dollar items an auction would be the best way to go as you will see the highest return on your better stamps. The lower stamps are going to be filler stamps that garner little interest at auction but your return on high dollar items would be maximized. The downside of an auction is the commission paid and the time it takes.
2: Outright Sale to a Dealer. Quick and easy BUT you won't maximize the value of your collection. That job will fall to the dealer as they have to house, inventory, display/advertise, auction/sell your collection starting with the high end stamps first. Because of the work the dealer takes on you won't get top dollar for your collection.
3: Consignment. Dealers may take better collections on consignment. They will house your collection at their shop or on their online marketplace. You will receive a higher price vs the outright sale option and the dealer will take a commission on the sale. This could be slow and painful if you want to move the collection quickly.
4: Private Party Transaction. Find someone like me who collects stamps (my focus: US Commemorative, Airmail, Special Delivery, Parcel Post and Ducks) and see if they might have interest in your collection. You might be able to find Facebook groups that deal with selling stamps. Those groups could open a lead to a private party sale.
5: Do it Yourself! Set up an e-bay or HipStamp account and have at it. If you are not an expert at stamps or haven't had them certified this can be a nightmare. Use this if you want frustrating, sleepless nights.
6: Donation. If your collection has more sentimental value over dollar value consider donating it to a non-profit or a local stamp club. Your stamps will find new homes in different collections and you get the tax write-off.
Hope that helps. If you have any question please reach out to us at rncstamps@gmail.com
RnC
(1) Certified Stamps offer buyers a piece of mind that experts have examined the stamps and determined them to be genuine. Stamp Certification Services can be found from the Philatelic Foundation, Professional Stamp Experts, Philatelic Stamp Authentication and Grading (PSAG), or American Philatelic Expertizing Service (APEX). I have certificates from all of them.