I tend to see people offering books on ebay that are not competitively priced or are so past what it can still be purchased for it's borderline silly
I order my new books from Midtown Comics - The Best Online Comic Book Shop - Buy Spider-Man, Superman, Batman Comics, Toys, more
Usually I can get issues for 2.15-2.45
Pre-order issues seem to run me 1.94
I usually buy multiple copies of a pre-order so I tend to save 1.00+/ea over what they cost me to buy anywhere else so buying 5 books or more usually offsets the shipping cost.
I search Midtown 1st for newer issues as well.
Secondly I search CCL Comic Collector Live - Buy, Sell, Organize Comic Books, Action Figures, Comic Art, and More
You will see some outright stupid prices here more often than not,but for filling older titles there are soooo many shops with .50 books and free shipping on purchases 20-30.00
I picked up 390 Uncanny X-Men for 183.00 dlvd. Not sure about you but I can price back issues at 2.00/ea and they sell,so it works well for me.
I tend to check for a title at all 3 locations,then narrow down the issue number and see where my best bet it.
the good thing about CCL is if a book gets hot and you ge on quick you can obtain books selling well for cover still,but you have to be fast
I know there are a multitude of places,but these seem to be most effective and beneficial for myself.