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If you want to solve this in practice, I would suggest that you formulate it as an instance of integer linear programming (ILP).

You can have a variable $x_i$ for each article that is one if we buy the $i$th article, or $0$ otherwise. Now you get some constraints. (If the seller has multiple copies of the card available, $x_i$ counts the number of them that you buy from that seller.) For instance, if I want 2 of the Dreadlord card, and they're offered in the 3rd, 7th, and 8th articles, then I get the constraint that $x_3+x_7+x_8 \ge 2$. The objective function is your total cost. You want to minimize your total cost, subject to the requirements.

Then, throw an off-the-shelf ILP solver at the resulting set of constraints.

This problem is likely to be NP-hard, so in principle there might not be any efficient solution... but in practice ILP solvers are surprisingly good, so I bet they'll give you a pretty good solution to your problem.