1) Always buy items with low price tags, i.e., buy things that you can afford with your "disposable income", so that if the deal don't go your way, you can rationalize it as a donation to charity (apply positive thinking always!)

2) Always check out the ratings of the seller!  Read the comments that comes with the ratings and try to make a judgement as to whether you think the person is trustworthy enough for your time and $$ and efforts.

3) Always compare prices, at least at 3 different sources, both online and in the traditional see, take and pay stores.  When doing so, make sure you are comparing apple to apple.  i.e. not all items with the same name are made equal (remember, 1 carat diamond ring can be worth many thousands or just a few hundred bucks depending on the Quality of the item!)

4) Check out the pictures and scrutinize them carefully.  In doubt, its always possible to ask the seller for more close ups or elaborations on the details of the merchandise :)

5) Always make sure you are clear what you are paying for.  If the shipping is going to be much more than the price of your item, you have either gotten yourself a terrific bargain, or you are about to waste money on shipments that are too 'safe' (eg. courier for some low value items are rarely justifiable)

6) Always work out the details of the shipping plan before taking the plunge and clicking "Buy now".  If you need international shipping, but the seller somehow cannot provide that, no point forcing.  If you need COD, but the seller don't do that on principle, no point forcing etc

7) If you can get a few more items and save loads on shipping, pool your orders with your friends, start bonding closer friendships with online-shopping sprees get togethers!

8) Searching is an important skill in online buying.  Make sure you exhuast your keywords and think of all permutations possible to ensure you get the best deals from what's posted.

9) If you cannot find the item you want, but found something similar, always msg the seller to check if they can get the stock for you!  Chances of success are very high!

10) Lastly, its good to get thing which will not post any health risk.. you get the point...