Ebay is a useful way of obtaining cross-stitch kits that are perhaps discontinued or are hard to find. I personally have bought kits via auctions because I don't know if I will ever get around to doing them and I did not want to pay the full RRP!
The secret is persistent and frequent searching. This may result in a saving of over 50% on the normal price of a kit even when you include delivery. If you are collecting a particular series and doing one at a time then you can pick which auctions to bid on and set your price limit (don't forget to use advance search to find out what similar items have previously made and how much your rival bidder has bid on the past for similar things).
Sometimes sellers only offer the chart only, if you have completed another kit in the series then they may share some thread colours so you may not need to purchase all the threads for a kit if only a small amount of colour is on the design. Don't forget to view sellers other items as you are likely to get a postage discount. If it a particularly hard to find item and you have been outbid there is no harm in enquiring if they have a second one and would the seller be prepared to give you a second chance offer.
Don't forget want it now adverts may be worth a try or seek advice from chatrooms or discussion groups.
Successful searching and happy stitching to you all!