Does any government agency or eBay department currently exist,
relating to real-time (not "after the fact") fraud prevention?
The jurisdiction in question is Brooklyn NY.

I have strong reasons to believe that a particular seller is
committing fraud on eBay right now, by selling items he has
no intention of shipping. 

It is very frustrating to see a crime occurring and to have
no way to stop it. I don't think I can call 911 on this one.

Any law-enforcement agent could place a winning bid on one
of the items and catch this guy in the act.

Hello atr, 

Here are some places where you can report crimes online:

FBI - https://tips.fbi.gov/

Internet Fraud Complaint Center - http://www1.ifccfbi.gov/index.asp

Note that the IFCC says that: "If this matter is related to a time
sensitive issue please do not submit information at this location." If
it is time sensative, they recommend that you contact your local law
enforcement agency.

You can find your local FBI field office here:
http://www.fbi.gov/contact/fo/fo.htm I suggest you give them a call
and see what they can do.

You might also want to consider contacting the United States Secret
Service as their mandate is to "Today our primary investigative
mission is to safeguard the payment and financial systems of the
United States. [...] Since 1984, our investigative responsibilities
have expanded to include crimes that involve financial institution
fraud, computer and telecommunications fraud, false identification
documents, access device fraud, advance fee fraud, electronic funds
transfers, and money laundering as it relates to our core violations."
SOURCE: http://www.treas.gov/usss/investigations.shtml

Again, I recommend that you contact your nearest field office as they
are more likely to act on it than the central location:
http://www.treas.gov/usss/field_offices.shtml

It could well be the case that they will attempt to catch this person
in "real time" but the decision will be completely up to them, and
will probably depend on what your information is like. If this is a
large scale fraud operation, it is quite possible that they would act
from your information.

I have found that reporting fraud to ebay results in the auction being
removed most of the time, but here is the link to the appropriate ebay
department: http://pages.ebay.ca/help/community/investigates.html

I do understand that calling 911 would probably result in them telling
you to contact your local law enforcement agency, but I think you
would have a better chance if you contacted the nearest CIA and FBI
location to you (by phone).

I wish you the best and hope that this information will prove to be
useful. It is nice of you to protect ebay members and I understand
what your frusteration must be like having been duped on ebay before
myself and finding that there was nowhere to turn except to do a
credit card chargeback.

Please let me know if you require any clarifications regarding this
answer and I will do my best to further assist you.

tisme-ga

Thanks for the info. I talked to the FBI field office. Unfortunately
they won't take a report from me because I'm not the victim...