From experience... quality.

Best -> buying directly from a dealer or online with detailed photos and descriptions (if they don't say its uncirculated, you can bet it aint).

Worst -> buying traders, chip lots, or chips with "fuzzy" descriptions whether from the board or online.

From experience... price.

1) If you are determined and patient, the best deals can be found at auction.

2) Advertise what you are looking for and let members make you offers.

3) Buy from a dealer at full price (However, this is pretty much a guarantee of quality chip).