This weekend we have Pro Tour Eldritch Moon and new decks are about to be discovered. We only have a few events of this magnitude per year, so don't miss the chance to invest. I strongly recommend watching the live coverage online because price spikes only last a few hours or even minutes, and price history tracking sites wont work for this purpose. I also recommend thinking twice before buying a card during the coverage; you might be tempted to buy cards of new decks faster than anyone else before the price rise, but not all decks will have a good record at the end of the day. And last, but no less important, don't be greedy, cards don't go up infinitely; if you have a good profit, it's usually better to close the deal early.
Some of the best cards to invest prior to the Pro Tour, now that we have the first peek of the new metagame after two StarcityGame Tournaments (with a great performance of the new Team Cardhoarder), are the underplayed good cards. Those cards are cheap at the moment because they seem to have no home in the new standard, but that is just until pro players use them in new decks.
Cards to Look For
Some of them might not be cheap any more for speculation as we are closer to the weekend, but if you manage to get them for a cheap price before the PT you should be good.
Ob Nixilis Reignited
Sorin, Grim Nemesis
Declaration in Stone
I didn't include a single Eldritch Moon, Dragons of Tarkir or Magic Origins cards because they are hard to invest in; if you are a beginner in the MTGO finance world just avoid them.
Have a great week!