I have amassed over 1000 legendary marks and so far only Xur has a use for them. I would love to see some other items cost legendary marks instead of always glimmer.
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Go to your vault and start buying as many exotics as you can from your collections. They will cost 10 shards. Then go to your character inventory and delete all the ones you just bought. They will give you 3 gunsmith materials and 5 legendary shards back. So basically 5 shards for 3 gunsmith materials. Hope this helps!