Ah, well.
It's all about question and demand. If you are going to "sell" for example LTC, you can fill in the price you want to earn. For example 0,03 BTC.
People who are looking to buy a LTC, can also fill in a price they are willing to pay. For example 0,025 BTC per LTC.
And there´s the gap. This gap will change because either some sellers are asking less BTC for their LTC, or the buyers are willing to pay more BTC for the LTC. And that´s how the marked moves each minute.
If you want your LTC sold fast, look at the buyers. If you see they are willing to pay 0,025 BTC for your LTC, ask 0,025 BTC for your LTC. The deal will be made in a few seconds then.
As i´m not a native English speaker, I don´t really understand your question ´Why can I not just accept terms of an open proposal?´. You can create an order, for example you want to sell 5 LTC at 0,04 BTC each. Just fill that in, and the order will stands for a long time. (Untill it is automatically removed by Cryptsy due to any reason or because someone bought it).
Besides, you can click on the offers in the sellers and buyers list. This makes the process of creating an order easier.
Did that help you?