I think Bidstart is also a good choice. The fees there are modest, so sellers, many of whom are collectors selling off their duplicates, tend to price things pretty attractively. Also, most sellers keep shipping fees pretty modest as well. It's very easy to search by Scott number there, unlike
ebay. There is also a good discussion board there where you can ask questions.
Having said that, I wouldn't want to discourage you from joining the APS. It does amazing things to help promote the hobby, puts on the best national shows, and publishes a great monthly periodical. But, personally, I never found the sales circuit books to be very helpful. When I was subcribed to them, they were very hit and miss, in terms of filling the holes in my collection, plus, when you are new to a circuit, you are the last one to get the books before they're returned to the APS. So things may have been picked over by the time you get them. You build seniority over time to work higher up the routing list. .. However, the APS also now has an online stamp store (they didn't have it when I did the circuit books), which I think is a more efficient way to shop than the circuit books. Shopping the online store also requires having an APS membership, but you could browse online first to see if you think it might appeal to you.
I would supply links to BidStart and the APS Stamp Store, but since punishment seems to be harsh here if you don't follow the rules, so I'm going to assume that I'm not permitted to post links.
(In the interests of full disclosure, I should add that I have in the past been a seller on Bidstart, selling off my Japanese duplicates. I'm paying to keep a place for a store there, but it's not open for business, for at least several more months, and I wasn't selling US or Canada. -- I know when I'm looking for a stamp, I look at Bidstart first.)
-- Dave