Lucas a word of advice from an old pro. Check the market prices for the cards you wanna sell and then add an extra few clintz . I would add 20 for the common, 30 for uncommon and 50 for rare cards. Trust me it works and do not sell for under the market value unless some cards do not sell.
I think the most helpful thing is to look at the cost of the card for the month. If the current going rate is higher than a month ago, sell. If it's lower, then hold it. Daily and weekly rates can fluctuate greatly but the monthly rate is a good indicator of the value of your card.