Uline is awesome if you live close to one and can pick orders up at will call. Also, you really need to be doing enough volume for it to be worthwhile.
We use Uline plus whatever boxes we come into for free from various means.
One thing I know of some people doing that aren't super high volume is buying the quantity of boxes that Uline requires, and then selling smaller lots of those boxes on eBay at a profit. It will take a while to recoup the $, but it's an option.
What I'd recommend with Uline is take a look at the things you have, and try to identify as few box sizes as possible that will let you economically ship your items and then buy those. For example, we only keep 5 types of boxes in any significant amount for shipping. If something random comes up we probably have a big one from an order or can easily get one. The majority of the items that my wife sells fit in a 4x4x4 cube, so that's the one we have the most of at any given time.
If you are doing enough volume that buying in bulk from Uline is worthwhile, then you also need a label printer. The one we use is a Dymo 450 Turbo, but we have a couple of earlier Dymos as well. No ink/toner and no cutting labels. Assuming you're using PayPal, the label type is 99019. At the moment we buy those in enough quantity that our price per label is below .03, which is actually way less than you can do it for on many other printers.