There's basically three main options:

1. Working on your own Collection. So picking one you're missing and that can't be cheaply grabbed in Sales or otherwise gotten (because the item shows up pretty often in events or you have a divine that drops it).

2. Selling for profit. So checking Sales first.
On the German server the default for Sale is the Ghost from Bewitched Pumpkin since some people spend quite a bit to get a small herd of them due to the (extremely low) chance of it maybe bringing back a GA or Wand from its excursions to Ow. Alternatively some of the Wanderer Spirits.

3. For a skiller, so one of the coats that give +14 bonus. Which those are is something you can see by looking at them.

The "rare" coats are generally for the older items (Wanderers, Witch , Hiking Journal, Sherlock, Inkwells have an evenly spread chance). With it being the far left coat on the page.
So Metal Element, Anciana, Violet Iris, Pumpkin, Shaman and Triton. Pestilence technically, but unlike the rest it never really showed in its Sale price. Generally with the loss of the old trophies and Howrse devaluing stuff all the time there's been a drop in worth.