I don't understand how Xür sales work.
I want a to buy something from him and I have the needed strange coins, the needed glimmer but he is asking one more thing: What I want to buy from him.... How is that ??? Do I need and engram or something?
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Thats called an upgrade. The stuff he sells is the top row, and the middle row is for lower damage exotics to be upgraded to damages higher than that of original exotics.
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That's to upgrade the item not to buy it. It's so you can upgrade old exotics to the new stats.