Speaking of secondary market.. It's champagne today : one of the biggest store online just purchased my whole stock of paradox engine, 16 copies. I bought them for 4-5€/each one year and a half. Between this, the 18 RIP, planar bridge, tolaria west, oath of nissa, gemstone cavern, anointed procession, as foretold, beast whisperer, razaketh and torment of hailfire i sold the past 3 months, i now have enough liquidity to make some fresh specs for early 2020 cash in target.
I'm still building slowly but surely my position on recently reprinted staples, namely ensnaring bridge, chalice, bloodghast, azusa, eidolon, urborg, runed halo,DS and VV.
But also buying for bulk prices cheap cards with big potential, like geier reach sanitarium, prized amalgam (still surprised the price didn't mooned yet on this one, wont take long desu, probably a matter of weeks, even days), eldritch evolution, fastlands... In general i love to spec on cards from eldritch moon, shadow over innistrad and amonkhet/HOD, they really are undervalued sets, and with time passing more and more gems from these will start to see play, like bedlam or Titi, who wee worthless not so long ago. Also they alrdy were success story for me (tracker&titi for 3€, collective brutality for 8€, anointed procession for 3€ and foretold for 5€).
Tho few specs also went horribly wrong : spell queller and selfless spirit didn't mooned as i expected it last year. As well as scavenger grounds, ballista (challenger decks caught me off guard), thalia lieutenant (human still could be huge but somehow everyone stopped to play it, even if GP datas show it still have one of the biggest winrate and the best day2 conversion... modern players are weirds) and thicc thalia (probably underestimated the BaB quantities out there for her).
Tell me the cards you think will make a good spec and i'll tell you the potential of the card for next year and long term.