Knowing the weigh of everything you intent to list is critical. I once sold about 400 Lucy & Me figurines and made sure the weight of each lot was under one pound including all packaging and the label. I have three scales very close by in my listing / shipping department. Two are digital scales and one is an old Pelouze model P2 postage spring scale with useless first class rates printed on it. One ounce was 34¢ at the time I bought that scale but it comes in so handy for setting my S&H rates.
I keep a print out of the USPS first class rate chart along with the domestic flat rate chart close by for quick reference so I have the ebay discount rates in view.
Why would you not print your own labels? Top rated sellers get great discounts on postage and don't have to stand in line, ever. At one point I was getting 17% off postage. Today it seems 11% is typical. That 11% is what pays for all my purchased supplies.
As far as dimensions go remember that most objects are three dimensional so don't forget to put the depth in your listings. It is good to take a birds eye view photo while your at it and even a shot of the bottom view helps on many items.
