Today I ordered ten Modern Master's cards from a vendor on TCGPlayer for $39.97 that buylist at $42.31. Generally I write articles that have actionable advice, but with this article I can only offer you this advice: use TCG's new seller policy to your advantage. After Star City Games' Las Vegas event I expect more individual sellers will list Modern Masters (MM) cards on TCG for below buylist pricing. I am also confident that individual sellers are listing non MM cards today on TCG equal to or under buylist pricing. While an influx of MMs cards might temporarily have a negative impact on current buylist pricing, I don't think that negative impact will be long-lived.
Perhaps more to the point, I have nearly a 6% cushion to buylist today. The reasons for loving market inefficiencies like this should be obvious, but just in case: remain liquid by acquiring cards at their liquidation value or less, reduce opportunity costs with this liquidity, and build up a trade inventory while showing a profit (assets greater than liabilities). One useful tidbit: check every seller's listing if you find a single card below buylist from them. All my purchases today came from a single seller.
Modern cards remain my favorite singles to acquire in this fashion at least until the next Modern season is in full swing, although occasionally EDH cards pop up under buylist on TCG - right now Coffin Queen is selling for a little over 13% of buylist.
If you want to invest in Standard at equal to or less than buylist, Ebay looks like your best bet. Occasionally an off hours auction ends for Steam Vents at <$4.50 or (foil) <$15 each. TCG remaining price efficient in Standard likely reflects a larger player base, meaning deals are snapped up more quickly. Rather than trying to get lucky sniping individual sellers of Standard cards on TCG and assuming Core sets remain under-drafted, buying boxes of M14 looks like the play as Scavenging Ooze is bonkers as a rare. I'd consider a reprint of Thoughtseize icing on the cake.
"I have a bunch of Modern stuff, now what?"
Hold them unless you are trading into Shocklands today or ((crossing fingers)) picking up Snapecaster Mage, Liliana of the Veil, and Cavern of Souls post rotation on the cheap. You are looking to trade into more Modern until that format is in full swing. Let Wizards and SSG do the heavy lifting of supporting and promoting the format while you trade sideways. Using inefficient TCG pricing, you'll have plenty of opportunity to trade up in terms of buylists while offering trade partners equal TCG value.
The catalysts for Modern specs are real easy to spot. Picking up cards late this month as supply from Las Vegas hits the market to sell into September Grand Prix events is a reasonable investment horizon that offers some wiggle room thanks to an October Modern Grand Prix event. Shocklands should rise with the rest of your Modern speculations as they'll likely be the backbone of many mana bases in the format along side Mirrodin Fastlands. Unloading Shocks while they see play in Standard and Modern could help push RTR and GTC reprints to RAV/DIS price levels.
With multiple vendors offering NM copies of Mind Funeral for sale at less than $2 while a major vendor is still looking for 72 copies at that price the bargain hunting has just begun (hooray, actionable advice!). Don't limit yourself to the easy tools, and don't limit yourself to Modern Masters chase cards. It's very likely uncommons and commons will offer the most volatility in pricing and present the nimble speculator with the most opportunity for profit over the next couple of weeks.