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.