Guild marks are a currency that you use to buy various items in your stronghold, like the powerful Dragonflight armor. You get guild marks from donating practically any items in the game to the mimic in your stronghold. Some things are more valuable than others when it comes to guild marks. You can either farm for your guild marks or you can buy vouchers and other items from the auction house to donate. Either way, doing this during a double Guild Mark event is preferable.
Farming Guild Marks
The cheapest way to get Guild Marks is by farming items yourself that you can donate. And the most stable way to farm it is doing Heroic Events in your guilds Stronghold. If you have a big and active guild, most guilds will do runs most days where they do the epic Heroic Events. The advantage of doing the epic ones is that they give additional rewards in addition to the influence that you will use to donate for guild marks. These rewards include resonance stones and companion only gear. However, all Heroic Events give the same amount of Influence and is capped daily at 400. The mobs in the Heroic Events also drop vouchers that can be donated for extra Guild Marks. The smaller Heroic Events can be soloed by most players. I recommend doing these daily for free and easy Guild Marks.
Running Dragonflight events with your guild will also provide you with vouchers, as well as the dragonfangs that you need to get your gear. Additional vouchers can be had if you are willing to spend zen to open the strongboxes.
Everything else like gold, equipment, and various other items can also be donated and farmed for Guild Marks but most things do not give a lot of Marks. Donating Labour is one of the few things you can donate for a bit more than many of the other things, so especially during 2x events, you can gain a lot of guild marks from either doing professions and donating your laborers or buying cheap laborers off the auction house and donating them. This is what I do on most my characters if I have some AD to spend, and I can usually get enough marks for all my Dragonflight gear without spending too much AD.
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