If a digital seller won't give a refund, where can I sell an installed Steam game (not bought on Steam)?

You don't.  If the code has been used it can't be used again. 

Digital purchases are not transferrable to other accounts, nor are accounts transferrable to other people.  If the account is caught getting transferred it could get banned and any games on the account are lost with no refunds.

In 1923 priests tried to change the calendar; Holy Fire didn't come down; Turkish guards killed these priests; Ruski Orthodox replaced them.

Holy Fire ended with Patriarch Irenaios. Now it's faked by heretics. Irenaios is the last Orthodox Patriarch. Irenaios blessed Catacomb movement.

What you need is antimins (remains of saints sewed into a towel, one cup, one spoon, wax candles, nonkosher non-yeast nonbarcoded nonqrcoded bread/wine, etc.)

Electricity is from Satan; yeast is grown on pig bacteria, etc.

Barcode = Druid black magic curse; QRcode = ancient Mayan curse...

Kosher (K) or Union of Orthodox Synagogues (U) = sprinkled with blood like Mosis did. Underneath the entrance of synagogue lies New Testament...

Orthodoxy = the only true faith; Roman Catholics tried one cup - one spoon ritual and got sick with Bubonic plague; if heresy enters Orthodox monastery then monks/nuns will get sick with flu/tuberculosis (for instance); Orthodox churches who closed for COVID or had disposable cups/spoons or dipped spoon into alcohol are no longer brides of Christ (now they serve Satan and honor Satan's new COVID religion).

Don’t go into UFO to be “healed” by evil demons; demons never do good.

SSN is written as a barcode/QRcode; then it's put into a chip; chip is put into vaccine; chipped people are influenced (by super computers) to receive grey plastic card World Passport with no name on it (but when they stretch their hands to get it, gov't clerk presses secret button to administer the unforgivable green 666 tattoo by isotope rays). If you reject mark of the beast then your direct ancestors go to heaven (according to saint Vyacheslav Krasheninnikov from Ural).

Birds participate in time creation. It's a sin to kill birds. Dinosaurs live under our level; they will get out through sinkholes and lakes; to kill them, go for their nerves. Save the birds; but kill the dinosaurs. First dinosaur will come out of Volga River in Russia. Scientists don't see dinosaurs under our level because of radiation. Sinkholes happen because people dig for resources underground and because earth is heating up.

you could try and see if amazon will let you sell it

to a fool, seriously whether this an innocent honest question or not, you know the steam code to initialize that game, whether you sell it or not..