I am an avid stamp collector. So Many different collections to choose from. 
It has been a popular hobby as long as stamps have existed.  I've been collecting since I was 6.
Have you ever tried to sell any for profit?!
Unfortunately all the stamps I have are used so not very valuable past the proof of paymeny cost. 
Are there in hard to find ones that you are still looking for to complete a collection?
I really want the Washington-Franklin issues since it was a huge change by having two silhouettes instead of the traditional singular.
How do you go about finding it? 
If I couldn't find it online I'd probably contact a university philatelist to see if they had any leads on where to find them.
Any other tips for stamp collecting you can provide?
Collect them for the joy, relaxation and calming effects, while not worrying about the minor details or defects.