[winkeltripel]

But they at least get a tax write off for a percentage of the value of the donation.

[jlokier]

No. It's not "at least they get a tax write off", because that implies a benefit, such as when paying profits to a charity you want to support anyway.

With these returned items, the seller just loses money and has a lower income than if they could resell them.

Their tax goes down because of returns, but that's just normal income tax because their income goes down.

If they didn't donate the goods, just threw them away, they would pay exactly the same tax. So there is no tax benefit gained from donating.