This really isnt a question for askhistorians, because this is a basic economics question. But in answer to your question, yes; people were selling at a loss.

Stocks are ONLY worth as much as someone is willing to pay for them. Which is to say a stock is worth what sellers and buyers AGREE on. And stocks only go up if there are more buyers than sellers. In the crash, there were more sellers than buyers. And EVERYONE was trying to sell. And in an effort to sell out quick, they lower their price, which cause others, in turn to lower thier price even more (as at the time there was faith the market would recover). Thats what caused the crash.

Hope that helps