Keep in mind that if you choose "instant sell," make sure that the quoted price is what you expected it to be. I made the amateur mistake a while back of assuming that the quoted BTC/$ price would hold for all of the coins I sold. Rather, the quoted BTC/$ refers to the highest bid.
That's not quite right regarding how instant sell works (or how it should work, maybe the bids changed right before you pressed "OK").
Lets look at an example, using an example order book of how many BTC different buyers want and how much they want to pay:
|$ Bid||Amount||Running Total|
When you get a quote for an instant sell, Tradehill looks at all bids currently up and calculates how much you would get. Above, you see the first buyer wants to pay $2.88, but only wants 1 BTC at that price, so if you sold 1 BTC, it would quote you 1x$2.88. If you wanted to instant sell 25 BTC, then 1 BTC would go to the first buyer, and 24 would go to the second buyer, making the calculated earnings 1x$2.88 + 24x$2.875=$71.88, an average $2.8752 per BTC. If you wanted to sell 45 BTC, the quoted price would involve all five sellers, and would get you even less per BTC. Then you have ~5 seconds to press OK to execute the quoted sale.
If instead you put in a market order to sell 25 BTC @ $2.88, you don't get a quote. What would happen is 1 BTC would instantly sell to the first buyer. However, the remaining 24 would be added to the 'Ask' order book for later sale. They would remain unsold until more buyers want to buy BTC at $2.88 or higher and submit new bids.
If you put in an order priced above any current bids, say at $3.10, no BTC would sell right now. Instead, your complete order will be added to the ask book, and would not sell until the market price went up to your price (and others sellers may have lower-priced sell orders ahead of you).
If you've been hanging on to your BTC this long (and seen the price you could have gotten for them go from 10->5->3), you may want to put in a higher sell order than the current price and wait; miner desperation to sell your hard-mined coins is what drives the price down.
BTW, an instant sell is essentially the same thing as putting in a normal order, but with your price as $0.001. It will sell as many BTC as you ask, no matter how low the final price goes.