...sticking with SPS since this is the SPS forum. The most bullet proof approach is to go after old school stuff that is still in demand and still has a somewhat high price. These things have stood the test of time (Camaro is still in demand where the Corvair came in hot and flamed out). Not all of these are super easy, but they also are not super hard. Some of the new stuff always does end up going into the classic category, so you have to know what to watch out for, but it mostly is stuff that is beautiful without trick lighting... like Fox Flame a few years back, Pink Caddy and some others. Most of the Multicolored Tenuis are hot right now and some have probably worked their way onto the classic list like Sexy Corals Orange Passion or Tyree Superman Tenuis which have held their value as well as any that has been around that long. Homewrecker and Jareds Lucky Charms likely will get there, but they are somewhat harder to grow - I have beautiful color Homewrecker under just 14k halides in my re-build thread - this one does not need trick light. The multicolored tenuis that have fell in value like WD that need trick lights and grow really fast will just be commodity type of corals in your area if they are not already... these can be fun, but don't pay too much for them.
If a corals has a Vivid or WWC in front of it and you cannot find a colony online, then it was just a hacked up mari or wild colony. Stay away from these. Nearly anything from BattleCorals or Rocky Mountain Frags does well since they picked and choose their stuff with a more discerning eye. Reef Raft can be hit or miss, but they have a lot of nice older pieces that you can find colonies of. Jason Fox can be hit or miss too, but the older stuff that you can find lots of photos of is the real deal. If you can find any Atlantis or Westside pieces, these are old school goodness. It is hard to go wrong with most ORA stuff.
Basically, what always has been hot always will be. It takes a good eye to see what current fad stuff will always be hot, but it is a combo of not being so easy that they grow like weeds and colors in any tank.