While I see the negatives of buying/selling items for real life currency (or Paypal), I also see the reasons why players resort to it despite it being against the Terms of Service. In the game's economy, there are far too many items that surpass the maximum amount of mesos traded / held by a character in one transaction, and this doesn't always have to be the case for actual stat-increasing equipment.
Historically and currently, there can be permanent NX items obtained through Style Boxes that due to rarity or supply/demand, will surpass the 10 billion meso range. Attempting to make a profit from these items (which Nexon had stated in the past was one of the focuses of Style Boxes being tradable initially to help players make legitimate profit from the purchasing of NX) requires either solely trusting in a player that they will trade them immediately after trading 10 billion mesos and give the remaining amount.. or resorting to selling the item via methods like Paypal, where it at least protects the individual somewhat in the cause of fraud.
I've sadly never had the opportunity of having items that were worth 10bil+ (or at least ones that I would consider selling), but even for myself, unless I knew the individual very well, I'd feel uncomfortable selling the item through normal means when there is far too much risk for the player of being scammed or taken advantage of. This is one of the main reasons why I enjoy continuing to provide the /insert free market site name here/, as it helps protect all maplers, regardless of their economical status in-game from malicious attempts to take advantage of others, or purposeful mis-pricing. Knowledge is power.
So to sum this up, I don't agree with the idea of players having to make item transactions through other means outside of the game, but I understand it and don't really hold it against them. Nor do I feel like it's that much of a problem that it's impacting the game negatively in any way. If the item is not exploited, legitimately created by a player, and the player has spent their own money cubing or otherwise improving the item, they deserve the right to make a profit from it without the risk of losing their investment due to a lack of functionality in the game itself.
Even in the case of a player receiving money through Paypal instead of the money directly going to Nexon, in most cases that money still finds it's way back to Nexon in some way or another; whether it's the purchasing of more NX (for that seller's shops), cubing, or so forth to make more pre-cubed items to sell for profit. I ran a poll in my server a couple years back and about 80% of players who sold items outside of the game said that they'd usually end up spending 60-70% of the money they receive back on the game again. To make things more realistic (depending on your server of course), if a player spends $300-450 to get a Frenzy Totem from a player who got one from Marvel Machine, it's expected that the seller will likely spend a good $50-150 on more Marvel Machine spins to try to make even more profit. That's still profit for Nexon in a large degree, especially when you consider the amount of items that usually go beyond 10bil.
[*] Scrolled/Enhanced Tyrant Equips
[*] Lightning God Rings
[*] Scrolled/Perfected Weapons
[*] Certain NX Items
[*] Frenzy Totems
[*] Other rare items from Marvel Machine
[*] And then bulk amounts of Prime scrolls.
That's all generating some form of revenue for Nexon on top of the money spent to (usually) receive the item in the first place. :)