I recently took two batches of things to consignment -- I'd picked out the best condition clothes from the donate bag.
First stop was Buffalo Exchange: My experience there was even though I arrived just as they opened, there were two people ahead of me with big Ikea sacks of clothing (professional pickers I think), and it took 45 minutes to get to the front of the line as the buyers very carefully went through each item and there was a high degree of 'keep' as the sellers really knew what BE was looking for. I enjoyed browsing in the mean time.
From my stuff they bought a magenta metallic mini skirt, my grandmothers black velvet super wide leg pants (too small for me, vintage), a flowery chiffon tassled huge scarf thing, and a weird hat. They said to come back in the fall with some beat up pea coats and another weird winter hat I had. They paid about 3$ per item (30% of resale price) so not much (I would have gotten more for store credit), but if there wasn't a line I would probably go again with anything bizarre and otherwise of little value.
Then I went to a local consignment shop that specialized in natural materials, they didn't need an appointment, and took everything I had in season in good condition that was cotton or linen or some scarves that were very boho. They marked them for around 20$ each, and if they sell it's probably worth it because it's down the street from me and many of the items were previously thrifted by me and weren't my style now.
What I gathered is like Shevia says that it really simply has to do with the current inventory of the shop and the season, so browsing shops for the look and labels they carry is useful, but also just holding onto things for the right season.