Just like League of Legends, Marvel Heroes sells Hero Unlocks for players to get their favorite characters. The price for these unlocks have been reduced when compared to what it was on release, some are pretty cheap while the more popular heroes are a bit more pricey. Each hero offers different playstyle and different sets of abilities. Balance in PvP might not be properly balanced but this is mainly a co-op game and since players can get hero unlocks dropped from enemies, it doesn't really give anyone a clear advantage.
After Heroes, players can pay to get costumes for the heroes they own. These costumes are purely cosmetics without any bonus stats attached to it. Different heroes have different number of costumes available with more popular characters like Iron Man or Deadpool having much more varied costume options than other characters.
The Items tab is usually the one that contain items people will say Pay to Win so I will take a look at the content more carefully. The first few items are mini pets, which are purely cosmetics. If you like little minions that follows you around doing nothing then you might want these, although they do cost a lot. Some of them are even more expensive than a few Heroes, which is ridiculous.
Holographic Crafter are just for convenience and definitely not P2W. So you can choose not to go back to HQ to craft, convenient, but doesn't give any gameplay advantage. And next we have...Boosts which are almost always not P2W. I always believe that stuff like exp or magic find boost in an MMO doesn't give an unfair advantage. So they spend less time to level up and get better item, but does that reduce other player's enjoyment of the game? of course not.
There 's also an item in the Item tab that's used in high level crafting. While this is somewhat ridiculous, the item itself can be dropped from enemies in-game. If it's available in-game it is just there in the store for convenience sake and thus I'm okay with this. However, the drop rate of this particular item is also a bit low so its presence in the shop looks like a way to get more money from those who don't have the patience to farm that item.
Next, we have the Fortune Cards which is something that I'm not really sure about. It doesn't cost really much, but the content is RNG and some player will be tempted to buy a lot to get stuff they want. Just like in other games though, this is of course purely optional and the contents are not going to affect your gameplay.
The last tab is Specials, which contains somespecial costumes and a multifunction boost. I don't know why these needs to be in a different tab but just like the other costumes and boosts, these are not P2W.
So, as far as i can see, Marvel Heroes doesn't sell anything that will make a player stronger compared to other players. They might look better, or level up faster but those aren't what makes a game Pay to Win. The stuff you can buy in the store are all completely optional and players can even get heroes or costumes from loot drops, which makes it pretty fair in terms of time spent vs money. And as usual, paying for convenience is not the same as Pay to Win.