Sorry, kevin504 is right. CSA #11. Worth maybe $15. If it had colored lines or even parts of lines on 3 sides near the edge of the stamp paper, then it would be worth a lot more.
Did you get a buzz when you learned that it might be valuable? If so, maybe stamp collecting is for you.
There are many rare variations of stamps that take a good amount of time to learn to identify. It isn't worth it to pay someone to look at each individual stamp and look for rarities. It's a loosing bet. Stamp collecting is mostly relaxing, but sometimes you'll find something that could be rare and valuable. It's kind of like scratch off lotto tickets in a way.
Every collector of stamps does it for different reasons. Some like the beauty of the artwork. Some love collecting every variety. Some get good enough to do it as a career as a trader. I like it for all of the above including learning about the history of how they were used.
Regardless of this experience, I would take chasa's advice and take it to a stamp dealer or preferably an experienced collector to take a look before you sell it. It can't hurt you other than delay you putting it up on
ebay again a few weeks later.
At least you learned that not every stamp in the collection you have is worth pennies.
Good luck in whatever you decide to do,
Ryan