My daughter has been into AG for a couple years. When she started getting into it I decided I needed to do something to off set the cost. We have actually made money on our AG collecting and selling. First look on craigslist for deals. This time of year is going to be very competitive for finding a deal. If you are buying second hand, summer is probably the best time to start looking for deals. Next would be ebay, but it is best during the summer slump too. I have found some bargains on both craigslist and ebay. I have bought lots sold a lot of the retired accessories and my daughter has got to keep the doll for free. I have also had success at garage sales. I found a Rebecca and Kanani for $40 each. My daughter wanted Kanani, but I told her I could sell it and make enough to buy two dolls. I made $200 profit on Kanani alone. If you are interested in selling pieces from lots I would recommend selling things individually. For example my Kanani doll came with sandals, necklace and panties. I sold these separately on ebay and made much more than selling them with the doll.
If you are wanting to buy dolls new you can frequently find free shipping codes. Right now there is one BESTGIFT until late November. The best code I have ever found was $20 off $100 and that was a few months ago. Look at the sales items and you can sometimes find good deals. I have gotten mini dolls for $10 each. There are also deals for Cyber Monday.
As far as accessories, think outside the box. Look for miniature furniture and off brands at garage sales. I really don't think that AG accessories are very quality. I would recommend buying off brand if you don't plan on reselling them some day. I would not recommend buying off brand dolls they just don't compare to AG dolls.
Just like legos, I look at AG as investments. When my daughter is done with them I know she can get a lot of money back and possibly make a profit on some items.