Not a lawyer and not a heavily researched answer.
You do not own the game, rather, you own a license to use the game software. The license will specify the ways in which they allow you to use the software, and they will typically reserve all other rights. This most likely includes the right to profit from game in any way. Your selling of in-game items would, in that case, be a violation of the license and they would have recourse according to the license and/or the law.
If they can show you profited from their property, they are probably entitled to compensation. There may also be relevant statutory damages for violation of the license, although I am less sure about that. I am way out on a limb here but by violating the license terms you might be engaging in copyright violation (since you are only authorized to have and use the copy if you abide by the terms) and therefore you might be on the hook that way; see e.g. https://www.law.cornell.edu/uscode/text/17/504 for an idea of what you might be looking at if they decide it's worth their effort to pursue.