re: Sending to other countries
I can usually fit 4 paperbacks in the Priority Mail envelope, 6 if they're skinny. A single book via 1st Class is done via weight if the package goes over 8 oz (a lot of books are around 7 oz + an envelope) costs over $9 to send to Europe, it's much cheaper per book to send a bunch at once.
That being said, I don't put any restrictions on my international moochers (except for excluding an occasionally super heavy book) and I decide on each one which is the most cost efficient way to send.
Another factor in using the Priority Mail envelope is that I can use their envelope, print the postage at home (including customs form) and drop off at the Post Office without waiting in line and 1st Class you have to provide the envelope, hand write the customs form and stand in line to pay for the postage at the Post Office. If it's close in price, I'd just as so pay the extra dollar or so and use the Priority Mail.
I think some of the requests for asking about international sending don't get through, I asked about 2 books from someone and they only responded to one request, so I had to resend the other one. (Most of the mail I receive from BookMooch is identified as SPAM from my IP)