Give it 3 weeks and you'll have more marks than you can ever spend. Once your gear is above 280 there is no need to buy any vendor gear. At the moment it seems like you'll have to farm mats again but I have a feeling that won't be lasting much longer, especially if you have 3 characters at 280.
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This. In the first week many decent players have at least one character around the 280 level so it makes sense that after 3 weeks the only desirable gear will be raid and trials weapons and armour over 280. I've had few blue rare engrams over 280 drop so infusing those will be good to around 290 maybe, beyond that marks will be useless except for breaking down duplicates?