Probably more the surprise for me comes from what some parts are worth compared to others.

The first thing you learn is that size has no correlation between price in many instances. While colour can play a huge role in the price of a part.

One example of size is some of the bulky pieces used for trains or aircraft. They are quite large compared to a standard Lego brick, but only list around the 20-50 cent mark mostly. Often never selling.

On the other hand there are a couple of different Lego Technic pins in Black which are quite small, less than the size of a 1 x 2 brick which can sell for $1-$3. Partially because they last time they were produced for a set was over 10 years ago in the colour black, and now only made in blue or tan. But sometimes it is easier to disguise a part in black.

But also to more directly answer your question, no doubt I had some parts in both directions. One part was the 1 x 1 plate which I stocked up in Black, White, Light and Dark Bluish Grey. These are supposed to be great sellers, but now months later still have a large portion of that stock. It sells, just a lot slower than I expected.