I know it's tempting to just guess and post items for what we believe they should go for, but if your goal is to make money with the RMAH then you're going to need a series of items you consider when posting. Here's 5 that I check whenever I post on any MMO, not just a real money auction house like Diablo. These 5 universal tips should help you when the game is released (this year?).
1. Demand
How badly do other players want this item? What do they need it for? How many do they need per visit to the Auction House? Demand will help you to determine the number of items you post as well as stack sizes.
2. Supply
How many other sellers are there? How easy is it for players to collect this item? Supply will help you to determine whether you undercut current auctions and by how much.
3. Cycle
Will supply and demand shift in your favor or against it? Cycles in the game, such as weekend players versus weekday players or vacation times versus non-vacation times, will help you to determine the timing of your posting as well as the duration of your auctions.
4. Profit Margin
How much will you make if things turn out ok and your item sells right away? What about if it comes back once, twice, or even three times? Profit Margin helps you to determine whether it's worth it to post your item at all, as well as whether posting your item will make you more than modifying it in other ways (like Salvaging gear for materials).
5. Overall Strategy
Are conditions right to be selling this item? Did you analyze the conditions by considering the balance of supply, demand, cycles, and profit margins? If conditions are right, you should post your item and make an informed decision. If they aren't, then you need to consider alternatives like hoarding the items for later or using them in other ways.
Keep in mind that none of these can be answered simply by looking at the current prices on the auction house. You also need to have a keen understanding of the history of the items in question as well as your overall strategy. This takes planning and research to work correctly.
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