IME, the ecosystem for buying Lego is not very mature.
I tried to buy a bunch of 44 specific lego pieces (the parts were listed from a book and fairly common) through Brickseek and couldn't do it.
Brickseek has a "Easy Buy" where you can add items to your list and have it find sellers, but it didn't work at all for me. Manually trying to find each piece is not worth the effort.