I’d try Ebay

eBay, Chairish (you can define local pick up if you don't want to ship and all..)..Craiglist, Nextdoor(that'd usually get you cheaper price. but might be more convenient). You can donate there too-on Nextdoor I mean. They have "free" section. You probably know,

Donations-Restore is great but the fabric section is usually very, very lacking. So I don't know whether it's good for your curtains or not..one wants them to be found and loved, you know?

Maybe it depends on a store though.

Custom is only worth something to you, not to someone else who probably have different configurations than you. Doubtful you'll be able to sell them at all, so better off to donate.

Try Freecycle.org

I know my neighbor just recently sold her treatments to someone with the same model home, on Nextdoor, do you have a neighborhood with similar models? Our builder also built similar houses in other areas, so maybe something to look into as well.

I had no luck selling window treatments bc they were specific to my needs/measurements. Donate is probably your best option.

Habitat for Humanity ReStore's set their own inventory, but they do have to accept building materials and fixtures. I'm sure your local Restore would be very happy to have these considering they could hang them on the wall and it doesn't take up a lot of floor space. Great thing is you can take the full tax deduction since you have the receipt and they haven't reduced in value. We recently bought 3 gallons of paint and it was the wrong color and we change the scope of a project so we donated them to Habitat.

Donations to Habitat ReStore are sold to the general public to generate revenues that support building affordable homes for purchase by qualified low-income families via a 20 or 30 year mortgage at zero interest. Donations are not reused in homes except for those that come from vendors such as Lowe's, Home Depot or branded stores.