When to sell it and how long to put it up for
If you see a bunch of low sellers in auction, you should hold off. People aren't going to want your market price product if they see cheap ones.
You might want to bid on the low sellers, for 2 reasons. So you can place a bid and when you check "my bids" every now and then, you might win it for cheap if you win the auction. Also, I've seen some people bid up those cheap ones to give the illusion that theirs is the cheapest. It's all strategy. What I recommend though, is that you wait till the cheap ones expire, then put yours up.
If there are a lot of high sellers, put it up immediately. Chances are, no one is going to bid on the high sellers, but they will want yours at market price. Put it for a short period of time, before low sellers come out.
A common mistake people think is that the longer you put it up, the better. More people will see it, and more people will bid. You couldn't be more wrong. People are impatient, they want the item NOW.
People are always looking at the main auction board. If they see it's up for 48 hours, no one's going to wait TWO WHOLE DAYS for their thing to come in. They'll find it somewhere else, probably cheaper. I recommend you put it up for 2 hours. If no one bids, put it up again. A lot of people watch the main board, so if they see it in the last few minutes, they might bid. Most bids are made within the last few minutes.
If you put it up for 2 days, you are stuck. What if someone wants it and is willing to give you a lot in a trade? You won't get your item back for days, in the mean time you lose the sale.
Plus in 2 days, you are subject to price fluctuations. What if the price starts falling? You can't get your stuff back to sell for a day or more, then what? You just have to take it cause no one wants it, or if they bid, they'll bank their money. Plus if you have the item back sooner, you can try putting it up in the trading post again, and maybe get some decent offers.