The Secret Service has strict guidelines about how this can be "legally" done.

I realize there is no better way to advertise your favorite note than by providing full, high resolution scans and photographs of both sides of the note. I love to buy currency from ads that have quality photos of the actual note I will receive.

In addition some ebayers have decided to make very realistic identical size copies of currency and offer them for sale.

I am not condoning nor am I condemning this activity but please be aware. 

From my perspective there seems to be little enforcement on this activity so I have provided the following information so you can be aware of what the US Secret Service requires.

If you don't care stop here!

Otherwise read on:

The law sharply restricts photographs or other printed reproductions of paper currency United States and foreign governments.

The law is the Counterfeit Detection Act of 1992.

It is Public Law 102-550, in Section 411 of Title 31 of the Code of Federal Regulations.

It permits color illustrations of U.S. currency provided the illustration is of a size less than three-fourths or more than one and one-half, in linear dimension, of each part of the item illustrated.

The illustration is one-sided; and all negatives, plates, positives, digitized storage medium, graphic files, magnetic medium, optical storage devices, and any other thing used in the making of the illustration that contain an image of the illustration or any part thereof are destroyed and/or deleted or erased after their final use.

That is what is clearly stated in the Law.

I have provided one example, of my own note, of what the Secret Service allows as a photo:

I have provided this for your information only and you alone are ultimately responsible for your own activities.

Plug "Public Law 102-550, in Section 411 of Title 31 of the Code of Federal Regulations" into your browser and you can see for your self. 

Good luck and happy Ebaying.