The best two sites for tracking real-time pricing are thepit.com and
eBay.  Although thepit.com specializes in just sports cards, the
trading volume for sports cards is much lower than on eBay.  Thus,
prices tend to be more volatile on thepit.com resulting in many users
transferring cards to and from eBay to take advantage of price
inequities.  However, due to Topps' recent investment in thepit.com, I
expect the upcoming marketing efforts will dramatically increase
trading on thepit.

Another site you might look into is eTopps.  eTopps (www.eTopps.com)
is the ultimate internet-age card portfolio.  Of course, it only works
with the eTopps cards but they are quickly attracting a following
because of the novel IPO concept and the ease with which you can
liquidate your shares (er, cards) directly on eBay without any
shipping involved.  Many collectors have had great success "investing"
in eTopps cards.  This could be the future of sports card collecting.
It will be interesting to watch as it unfolds.