If you want to be sure you're investing in the right items, you need to know what items are limited in the game today. It's easy, though, and you don't need to ask 10,000 questions. The rarer an item is in terms of copies, or the fact that it can no longer be obtained in the game (prenerf), the higher its value is likely to rise over time. Taking into account the time when you could duplicate items. How many Pandas are available today (?) good question, at least not the number Arena created for the players who earned them back then. (But nothing's impossible... who knows if a duplicate doesn't exist today... let's not forget that GW2 also had duplicate)
The real question is: how much do you have to invest, because these items are expensive. 1k armbraces? 10k armbraces? more? If it's just a few stacks of ectoplasm or one stack of armbraces, it won't be enough. And given the demand and supply, you'll never be able to keep up with the botters who've amassed far more than you over the years. If that's the case, you've already lost out on before starting your investment.