Edit 12/03/16:
Let's address some of the people claiming that control groups aren't necessary in this kind of test. The case being made is that in a comparative study such as this, the white hand is the control and the black hand exists as the comparison. This model could work assuming there was a normalized distribution of resources between them (the only time uneven distribution couldwork would be with a much larger sample size but then you'd have to address other problems and certainly have this single paper as part of a longitudinal study). However, in this instance I think a control with no hand (or an ethnically unknown hand, possibly something gloved or an artificial hand) would still be useful.
If you are attempting to prove racism and you find that hand to no hand has a difference of some 20% (random number) and then the difference between black and white is only 1% that difference would indicate that there are other factors that are more important than racism and it would throw into question the claim that race is the most important factor (as opposed to other factors such as auction time/length, listing time and many other factors that need to be controlled for in a fluctuating free market; number of buyers, demand for product, amount of competitive units on the markets etc). Also, if you had a control with no hand and the white hand scored consistently or on average higher and the black hand scored consistently or on average lower that would serve to strengthen the case of racism because white hand good, black hand bad.
On a quick note about skin colour, it should also been noted that another way to make this test even more accurate would be to use multiple hands across a full spectrum of skin colours, depending on what source you use there are many different colours/tones (the Luschan Chromatic scale suggest 36 for example) , not a simple binary which can actually result in some people from certain ethnic groups over lapping each other in colour.
Does the lack of no hand control totally invalidate this kind of study by itself? No, but when compounded with the other problems it is only another damning point. Would it still be helpful? Yes, in my opinion because it would give greater scope to subsequent findings but feel free to disagree, you're not wrong to. I can't help but think this was ignored because they knew the difference from hand to no hand would have dwarfed the 'racism' problem and made it more laughable than it already is.
/edit
Junk science is junk and not even remotely science. At this point some social "scientists" seem willing to post any old shit as long as they can manipulate it to help perpetuate their chosen narrative.
People who clearly didn't read the paper talking about the paper:
https://www.washingtonpost.com/news/wonk/wp/2015/12/11/whites-earn-more-than-blacks-even-on-ebay/
http://fusion.net/story/244173/researchers-found-whites-earn-20-more-than-blacks-on-ebay/
Donations are greatly appreciated and enable me to acquire the delicious berries I need to live:
https://www.patreon.com/TealDeer
Copyright Disclaimer Under Section 107 of the Copyright Act 1976, allowance is made for fair use for purposes such as criticism, comment, news reporting, teaching, scholarship, and research. Fair use is a use permitted by copyright statute that might otherwise be infringing. Non-profit, educational or personal use tips the balance in favor of fair use.
20:52
