When I started making gold it was mostly out of the need to repair my gear. This was long before repairs could be charged to the guild bank, automatic deposits into a guild fund, etc. After it was obvious that I could earn much more than what was necessary for basic needs I was able to look at toys and items that I thought were neat.
Fast forward to the current gameplay where if you combine the black market auction house, regular auction house and even buying heroic mount runs just about every mount in the game is available to you if you have enough gold. Selling gold mount runs is a hefty income maker for many hardcore raiding guilds. Spend 20k+ in gold and you get a mount. Oprah would be proud.
This isn't a condemnation of this practice. You earn the gold, you should spend it how you like. It is a personal choice on whether you want to 'earn' the mount with your guild or just buy it. I decided a long time ago that I would prefer to earn it or not have it. Buying it would always be a reminder to myself that I wasn't good enough to be a part of a group that could finish the entire requirements for the mount.
The farther that we get away from many of these old world raids and instances the more apt that people will be buying and selling these mounts. And it still will be a money maker with the next expansion. Selling level 100 runs into Seige of Orgrimmar. Even older raids will be able to be soloed quickly for cash and prizes. Bringing along someone who isn't as geared but wants an awesome mount for 5k will add that much into your pockets.
Mount/achievements runs and selling them have always been the area of heavy guilds. But picking up the lower level raids and selling spots can add some into your guild and personal coffers. Even enough to buy your way into a nice mount run of your very own.
Good Luck and Good Profit Hunting
Want to get in contact with me? . Email me at Dragonbear @ gmail. Dot com or find me on twitter at "@JMTCMommar" You can find me hanging out on the Shandris Server, chat me up, and I will buy you a glass of juice.
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