We had a garage sale last year where we sold all my daughter's clothes ~ from 0-3 months - 3T. What I did was first separate all the clothes by size, then I folded them and pinned together everything that was an outfit and then I laid them out in tables in neat piles. Underneath the piles, I taped a piece of paper to the table stating what size it was. If there were more than two sizes on a certain table I would separate the clothes with masking tape and then make sure I put a sign underneath that part stating the size. Then I bought some posterboard and wrote down what each item was. For example, all onesies were 50 cents, all pants and shirts were a dollar, etc. I did individually place prices on the outfits or dresses because a lot of these were really nice, but most of them were no more than $3.00. I also put all the shoes on one table and marked those as $2.00 unless there were nicer ones, then I individually marked those and put a sign underneath stating that all shoes were $2.00 unless otherwise marked. I hung up the posterboard with the prices on the back wall of our garage and also in the front to the side as people walked up. We got a lot of compliments on how neatly organized our garage sale was. I had one lady spend about 2 hours going through all the clothes and bought two big bags full of clothes. She just quoted me a price and I just said that was fine. It was probably off by about $5.00, but I didn't care. We also ended up selling almost all of the shoes, too.
As I am sure you can imagine, it was extremely time-consuming, but in the end I believe we definitely made more money than if I were to bring all the clothes to Once Upon a Child or to a consignment store.
Good luck!