I don't believe bonds can't be traded through Zecco. Did a quick google search, if you want to buy bonds try these guys:
http://individuals.interactivebrokers...Fee structure for bonds:
http://individuals.interactivebrokers.com/en/accounts/fees/commission.php?ib_entity=llc
Most people on Zecco tend to be inexperienced, and deal in small amounts of money (I know I am). Bond funds offer severe disadvantages over purchasing the underlying bonds, but since bonds usually come in minimum $1k lots, often more like $10k, diversification is next to impossible.
I like/own BWX, which is a basket of foreign treasury bonds. There is also BND, Vanguard total bond market, EDV which is just US treasuries.