I have the non-B visa single entry. Now I am going to apply for the re-entry permit. My question is, do I have to buy only one ticket to Thailand or do I have to buy a return ticket?

With a re-entry permit you will have a valid permission to stay (if you return before your current permission to stay ends!) and do not need a return ticket back to another country.

You are in Thailand on a 90 day visa entry and plan a short trip so you obtain a re-entry permit so when you return you use that to get the same permitted to stay time you had before departing. It acts as a visa so there is no requirement for onward travel documents (as would be the case with a visa exempt entry).