First, don't deal in names in which you have no clue as the the marketability or capacity of the market to pay. Buy in fields you are familiar.
Second, It's better to take too little and still make a profit, than ask to much and not afford renewals. Don't worry if a more savvy dealer then resells for another profit. Once you know the market you can take that position. Sell, Profit, Forget.
Know the alternatives. If you have a name for sale for $XXXX and someone else has an acceptable name for $XX or in another TLD, you may not get what you wan't but send the buyer away. Know what alternitives names are available and what registered names are being used for (popular site, parked, PPC, 404)
Research the capacity of the domain's potential market to pay. A huge company could throw 5 figures at a usesful domain without a blink. Don't think a real nice name for a personal website or mom & pop store is going to draw the same money.