I will just demonstrate one...

I will just demonstrate one merchant:
https://yourvictoryvault.com/
https://yourvictoryvault-verify.isignthis.com/landing
On this site, various in-game currency are the sold.
You can inspect the rest of the games yourself, but here are a couple:
World of Warcraft
https://www.blizzard.com/en-gb/legal/fba4d00f-c7e4-4883-b8b9-1b4500a402ea/blizzard-end-user-license-agreement
Clause 1.C.iii, subclauses ii) and iii) as bolded.

Prohibited Commercial Uses: Exploit, in its entirety or individual components, the Platform for any purpose not expressly authorized by Blizzard, including, without limitation (i) playing the Game(s) at commercial establishments (subject to Section 1.B.v.3.); (ii) gathering in-game currency, items, or resources for sale outside of the Platform or the Game(s); (iii) performing in-game services including, without limitation, account boosting or power-leveling, in exchange for payment; (iv) communicating or facilitating (by text, live audio communications, or otherwise) any commercial advertisement, solicitation or offer through or within the Platform; or (v) organizing, promoting, facilitating, or participating in any event involving wagering on the outcome, or any other aspect of, Blizzard’s Games, whether or not such conduct constitutes gambling under the laws of any applicable jurisdiction, without authorization.

The Elder Scrolls Online
https://account.elderscrollsonline.com/terms-of-service

Virtual Currency is not transferable to, or redeemable for, any sum of real currency or monetary value from ZeniMax or any other person at any time. ZeniMax prohibits, and does not recognize any purported, transfers, sales, gifts, or trades of Virtual Currency. Virtual Currency will only be used by You to obtain virtual goods within a Game (e.g., durable goods that become part of Your inventory in the Game and consumable or single use items that are used and expire after use within the Game) or other Downloadable Content, through one or more Services, as determined by ZeniMax in its sole discretion. You agree not to sell or transfer, or assist others in selling or transferring Virtual Currencies or any amounts thereof. Evidence of any attempt to use, sell or transfer Virtual Currency in any manner that violates these Terms of Service may result in revocation, termination, or cancellation of the Virtual Currency and/or Your access to or receipt, play or use of the Services without refund and/or immediate suspension or termination of Your Account