So, as a QS member you have full access to trader tools, a backlog of insider articles, and a solid forum. While it's relatively easy to find advice about what you should be picking up and what you ought to let go, I think I can still offer some good advice about how to spend money in the MtG secondary market.
Buy at dealer prices for singles. You've got some wiggle room if you are willing to trade with shops that offer a bonus on trade credit. This is not hard thanks to Trader Tools. That handy tool often shows negative spreads between buy prices and tcg prices. Ebay remains a valuable source as well - yes that's a risk free Time Spiral.
While Ebay is easier to exploit thanks to easy access to ALL of a particular seller's listings, you'll benefit from focusing on the same sellers on both sites once you've found your mis-priced product. Simply put: the guy selling stuff too cheap isn't doing that once.
Buy sealed product that sells for less than expected value. That is not as hard as tracking down that elusive Mind Seize. Try the complete set of Planechase 2012 in Spanish for $84.49. You can sell parts to capefeargames for $40.47 after taking a 25% hit because they are Spanish. Alternatively on Ebay, the Night of the Ninja sold for 56.99, and a playset of Spanish Shardless Agent sold for over $60 shipped. For some reason, Ebay is also the best place to pick up complete sets of all four 2012 Planechase decks. Spanish sets there can be had for less than $75. That looks pretty low risk.
Once you've bought at these lows, you can feel much more comfortable trading into staples no matter the next Pro Tour format. Goremand at $16 is pricey, but trading into them will add velocity to your binder. That card is a great tempo play for Modern.
If you are more conservative, trade into the plethora of standard playable modern/legacy staples. You could do worse than Thoughtseize, Deathrite Shaman (is this banned in Modern yet?), Ashiok, Nightmare Weaver, shocklands, or Scavenging Ooze. While Mutavault looks pricey, much like Scavenging Ooze, it benefits from being reprinted in a core set. I won't be buying any but trading into them is still justifiable.
You should always be hoarding your MtG cash. Players often demand more value in trade, and if you are going to buy cards you need to be sure you leave plenty of room for profit when you trade those cards you've purchased. If you are going to try and earn an income through MtG speculation the reasons for tight money control should be obvious: the easiest place to unload your cards will always be dealers. Buying at or below their buylist prices offers you an easy path to profitability through trading with little to no risk of capital. Buy cards to trade into cards. Trade into cards you plan to sell. If you aren't doing both your hoarding. Make money not inventory!