At the moment most stocks are really quite thin after the normal close. Something such as Intel is still 1/16 wide, and you can trade it all you want, but the less liquid stocks may well not trade at all. Many of the Nasdaq stocks just have a couple of quotes very wide, and with nobody actively trading it you will not get filled on a limit order (unless you are hitting the bid/offer), and a market order is not advisable. The same for NYSE stocks, except I think that they trade on the ECNs after hours.
I don't know how the discount brokers work in afterhours - usually they sell their orderflow to a specialist, eg on the Chicago Stock Exchange, but I don't know who executes their orders in the evening. Nasdaq stocks will just go onto their ECN as usual.
Callspread