poly mailers on amazon are your best friend. Those and a food scale. Weigh it and print a label of USPS to save a few cents or just take it to the post office. Anything over 13oz has to go priority. The padded flat rate envelopes are good too- you have to order those online though. You can pack a lot in and they are $6 to mail I believe.
I'm bad at pricing but I'll look at other people's posts to get a rough idea.
I usually do prices without shipping and add it on, USPS website has a calculator that's easy to enter zip code and box weight, or you can use flat rate boxes which are nice too.
Also you may want to make more than 1 thread depending on brands. So one thread for gap/gymboree/ON/carters and then another thread if you have a lot of mini boden/HA/tea/J&J.
Do whatever @marshmallows and aspentosh say and I'll give you all my money
Are you having another girl Amy!?!
Another newbie tip - price everything plus shipping because the shipping can vary from location. I would go to a post office and get a couple flat rate mailers/boxes to have on hand and then you can throw a bunch in a box/bag and not have to worry about heavier things.
We held strong and are staying team green again! I just buy all their stuff for the DD I have now, who (so far) has been consistently one size behind their girls (But I will admit to having my fingers crossed for another DD!)
Posting either by similar items (long sleeve shirts, pants, dresses) is great. I have found some people like one item per picture and that makes it easier to note what is sold (just delete the picture).
They pay by paypal, and you can ship that way since it's cheapest. Mailman will pick up your packages for free.
I am happy to answer more questions, but those are the basics!