Separate the Used/Unused.
Separate by Country
Get in order - Date of Issue/Scott Number
Some Libraries have Scott catalogs you can use for FREE.
Remember that getting Catalog Value for common stamps won't happen. USA this side of the change to 3 cent stamps will mean you're lucky to get Face Value. ( 3 cents = 3 cents )
Common First Day cancels sell 4 for a dollar, or less.
If the subject of the stamp is not common, neither will be the interest. I've had Marilyn Monroe FDoI Ceremony Program for sale on eBay
for a Year or so...
Selling on eBay
or other sites means OVERHEAD COSTS! eBay
+ Paypal take their cut first, Shipping/Handling/Insurance/Tracking/Ect costs are placed on the Seller. If you have to add these costs to the price your asking for whatever stamps you're trying to sell, you won't make much money.
The time and effort it takes to create a valid eBay
lot may be quite a bit, especially if you're not familiar.
With all this in mind, Good Luck.