It may be worth removing the chainring. Sometimes they are warped because they were screwed down unevenly. Sometimes they are just bent and it is easy to see and correct when you place them on a glass or granite counter. And sometimes the problem is the spider arms not being true, which you can see by removing the rings, holding the seat tube and using your finger as a runout measure - all 4 or 5 spider arms should brush the end of your finger equally as you rotate the crank. I've seen all three on new bikes.
Being able to describe where the problem lies will avoid getting the wrong solution from the seller.