i would not recommend that you approach the dealer in any manner that could be perceived as hostile or aggressive.
in fact, he might not be obligated to take the machine back and give you a refund. this is especially true in the case of used machines. they are commonly sold "as is".
i am not suggesting that i believe he would be right to refuse a refund. i'm only pointing out that he might be legally free to refuse.
the suggestion that you take the proof/sample of the problem with you is excellent. i would also take along a good supply of sizable scraps to use while showing him the problem(s) you had when you tried to sew with it.
it might be that you need some pointers in proper use of the machine. it might be that a tension setting needs to be tweaked, or that he goofed up somewhere along the line when he serviced it before putting it up for sale.
he would be rather foolish to just blow you off. after all, he will want to be your first choice if/when you decide to purchase any more machines in the future. in a case like this, most dealers will not give a refund until they have made every possible attempt to solve whatever the problem is so that you take it back home in good working order.