How much oil was in it exactly ? Was it near the empty line ? Or barely on the dip stick ? Did you push dip stick in fully ? They are a bit tricky. I would check oil properly then gather all my evidence if their is a problem including all conversations in writing between you and the dealer, paperwork, bank statements, adverts, his registered business number, then inform trading standards of your intent to open a case. They will advise you to talk to the dealer first, do this and tell him you have opened a case. He may buckle, he may not. Then go from there.