Should I keep everything sealed ?

[Iwatasan]

If you only collect one franchise (like TLoU), it's definitely a do-able strategy. If you undertake it, it's probably best to buy the stuff most likely to increase in value first (before it becomes prohibitively expensive to buy sealed), and work your way down to the most common stuff that is least likely to increase in value (or that is likely to decrease in value so you can get it cheaper later compared to buying it new now).

I can definitely understand the desire to diversify your collection though, especially since you're just starting out. What kind of merch are you after? And what regions?