It's possible, but the default settings aren't setup to do it, because people tend to use rules that don't work the way they think, or miss things, then complain about the bot selling stuff they want to keep.
The default "Save" filter is called "Currency, uniques, and skill gems" and will make sure no "Sell" filter tries to sell currency, uniques, or skill gems.
If you update this save filter and remove the "Uniques" rarity from it, then you will be able to sell any unique.
However, if you happen to have a filter that matches other uniques you'd not normally want to sell, they'd also get sold because you have no backup filter in place to keep the bot from selling it.
To sell an unique by name, you'd add a Vendor filter that matched:
Rarity: Unique
Identified: (checkbox, not dash)
Names: enter the name of the item you want to sell. if you have multiple, make "Match Any" a checkbox.
If you know exactly what you want to do and are sure you're filters are perfect, you can modify the Save filter to not save all uniques, then add in a new rule to only sell the unique you want.
