Ultimately (obviously) the cards people want to play with will be worth more than their unloved counterparts. As speculators we must be able to accurately evaluate a card's playability to enjoy success. Figuring out what is going to be good in a Standard format is probably harder than doing the same for any other format. Don't believe me? Take a look at pre-sale prices for singles and compare that to prices after Game Day.
With a small card pool, Standard specs are best made when cards can be seen fitting into or strengthening decks that already see play. Evaluate a card only on its ability to replace pieces of a known commodity and you will be much less likely to be holding onto losing positions. Occasionally a new keyword can make some cards still in standard suddenly very playable. While this is rare, when it does happen it seems obvious. Devotion is the latest keyword to prop up some otherwise borderline cards (Nightveil Specter), but the ability for you to take advantage of devotion friendly cards hasn't totally gone away. A card like Boros Reckoner was played before and after the devotion mechanic debuted. Demand for this card is steady with decent upside should red or white devotion become more of a thing in the format.
Picking the right Modern cards for speculation is easier. A list of proven archetypes exists from which both new cards can be tested against and old cards can be found. Determining whether a new card belongs in an established decklist requires the same question a card vying for play in standard must answer: what am I replacing? Finding cards for Modern can also be a process of revitalizing, reinvigorating or strait up resurrecting an outmoded/ outclassed deck type.
Banning or unbanning a card often has that effect. The recent unbanning of Bitterblossom being an obvious boon to all things Faerie in Modern. More subtle is the effect of Wild Nacatl's return to the format. That move combined with the removal of Deathrite Shaman from the format should make Knight of the Reliquary decks much better and Faeries won't like the addition of Voice of Resurgence to decklists that already offer better card quality.
I hear some of you asking, "What about Birthing Pod?" Well, yeah, that card has been doing work as a functional replacement for Stoneforge Mystic. It also does something any card you would consider specing on should do: encouraging you to play with other good cards.
When thinking about how a card will impact a decklist, you will find cards that push you to run sub-optimal stuff. You've just found a very good reason not to put money into that particular card. For those of you wondering why I linked a Channel Fireball article or too lazy to click thru:
"...it was interesting that Brian Kibler, in describing the “Birthing Pod” deck he used to good success at the Invitational, said that he almost ditched the Pods completely. He didn’t, because there was still good synergy between the Pods and the other cards in the deck. But he did go down to two copies of Pod, which says a lot about what was most important in the deck."
Legacy remains a popular format in spite of huge barriers to entry. I love picking up cards leaving Standard that see play in one of Magic's most powerful formats. If it's played in Legacy, it's worth a look. Ignore rarity here. Innistrad was a very popular set, opened by many new players as the game we all hope to make some money by continued/s to grow in popularity. How many copies of this guy do you think exist?
While Innistrad block cards are still on sale thanks to rotation, Delver is still worth more than half a dollar. Gotta think that card is worthy of loading up on while it's still sitting in Standard player's binders begging to be appreciated. Something like Devour Flesh is going to be worth more than a bulk rare forever, so why not make some Johnny's day by trading a playset of his pet card for a playset of legacy playables?
The vintage cards you'll be likely to spec on are also legacy cards. Something like True-Name Nemesis is a no-brainer, making a spec on that card a matter of finding unopened Commander Decks. Of course, coping that Nemesis with a Phantasmal Image continues to make more than 5$ worth of sense.
Everybody's favorite casual format, EDH, can justify a lot of terrible investment theses. Rather than consider a potential card in terms of its EDH appeal, I consider similar cards in terms of their availability. Somewhat like traders demanding more for dual lands or other legacy "staples" picking up something in limited quantity if not total value that you aren't afraid to sit on is a fine way to dabble in the format. By making sure the cards I have for EDH players are perceived as limited in quantity I can justify asking for more should anyone show interest.
While I would never recommend speculating on EDH, foils or limited (alpha/beta) edition cards, I must admit there is nothing as satisfying as turning a foil Mercadian Masques Bribery into multiple playsets of various shocklands. Speaking of Bribery, with abugames offering over $68 in store credit for a NM copy buying one for $59.99 off tcgplayer could make sense assuming you can trust a 95% approval rating.
When considering all these formats, Modern seems like the best place for card speculators. The ability to look over a developed history of deck archetypes when looking for potential investment opportunities, and to make multiple bets on what is invariably a more diverse range of deck types compared to Standard. Although a Modern speculator doesn't have a reserve list to protect their investments, Wizards has shown much more interest in developing this format.
In addition to offering Modern Masters to help stabilize pricing (without crashing the secondary market), Wizards has announced a Modern event deck will also be made available for sale end of May. The hefty price tag relative to other event decks again suggests a desire to offer "staples" to the community without destroying the value of current player collections. Finally, it seems likely to me that not all the new players coming to Magic want to hold a bunch of "useless" cards. A far more natural progression would be for those players to move into a format that allows them to play with all their cards - including their favorite cards no longer legal in Standard.
I'll conclude with a brief summary of my methods for card evaluation. First, before buying ask what this card will be replacing in an established archetype. Along similar lines, what cards does my potential spec target demand see play along side. Often a card will be perceived right away as a potential foundation for a new archetype. When considering a card like this for investment, you must be sure you've a body of cards to complete the picture. Finally, look forward to rotation. This is the easiest time to pick up Legacy playables on the cheap and to a lesser degree the same can be said for Modern playables. Ignore rarity when considering playability. If a card is played in Modern AND Legacy it will likely rise in value post rotation slump as it moves further and further from its time in Standard.