For organization -please keep 1 goldenrod envelop with clasp for receipts and another for returns (properly labeled). The color of the envelop helps me find them quickly. The size of the envelop typically fits in most purses.
www.Ebay.com - I typically look for a price point at about 1/2 of the store price (including shipping charges). For my own use or for kids, slightly used is ok.
www.Amazon.com - variety, deals...look for deal codes or scenarios on money saving blogs.
www.Overstock.com - great comments from people who have purchased items.
www.napervilleyardsales.com - local pickup/payment (mostly moms, friendly, and clean items)
www.craigslist.com - I have used and have felt comfortable with the transactions.