Hmm- I think it depends. There are more popular patterns like Butterprint, Snowflake, Primary, Crazy Daisy, and Gooseberry, and then there are rare patterns like Nell said, which are the promo ones. I sell more of the common patterns than I do of the promos- I guess it's because people choose to collect common patterns and then they want the last few pieces that they can't find for whatever reason.
Also, certain pieces like the refrigerator dishes with lids are harder to find than say, a mixing bowl. Also, look out for butter dishes, with lids.