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.