$20 Dollar Bill/Note w/a * Watermark Error Covering the Entire Rear/Back of the Bill/Note...

As we discussed on the phone, it's hard to tell how dramatic it is from the photograph. I'm not sure to what degree it's a watermark v. bleed through or transfer issue. That there might not be much for reference because it's unusual doesn't necessarily translate to value. My estimate of value is $50-200, wide because errors are a collecting niche. Your idea of doing a fixed price listing on eBay sounds good, start high and you can always drop it periodically like a Dutch auction until you get a buyer.