The most basic tip that i can give to any coin collector is:

Purchase the best coin that you can afford!

The better grade coins are the ones that appreciate in value faster than common grade coins, (this info is mainly for investors)

more added 03/10/07

I thought that the first review was a little too short so i wanted to add more:

1. always buy from a reputable dealer, this is kind of hard on ebay, but use the feedback system before bidding,

2. if it is too good to be true then it probably is a fake, if you see a 1909 wheat cent s vdb selling for under a hundred bucks, it might be a fake!!!!  there are a lot of bad people out there, but hopefully ebay dont let them sell

3. try to buy from people with a return policy, that way if the item was described wrong, you can return it.

4.  you cant go wrong with gold, silver, or platinum, these coins are always worth their spot prices, if the coin is also rare or of a rare mint, that only adds to the value, but that dont mean pay 1000 bucks for a 1 ounce gold bar if it is only worth 650, since it is not a coin, it only has its market price backing it, but if it was a coin (i refer to mainly u.s. coins) then you know that it was produced in limited quatities and in different grades.

5. if attending a coin show, carry a magnifying glass with you to closely check the coins, if you find a ms70 coin but with you magnifying glass you see little scratches, you know that you are dealing with an inexperienced seller or a crooked seller, in both cases you should point out the descrepency to the seller.

hopefully the new additions will help you when purchasing coins on ebay or in real life, i apologize for any misspelled words, if this helped you or you think something needs to be added, please drop me a line!