What "items" are you talking about, because I don't understand what context this is. Sex? Money? Gaia Items? I'm confused.
Regardless, the cousin obviously found out from your ex about the items. Or through the grapevine. Anyway, it depends on what your present relationship is with the cousin and the ex. Are you still chums with the ex? Are you good friends with the cousin as well? What about the cousin and the ex? If they're not that close, it is suspicious that the cousin found out.
I personally wouldn't trust either of them. If you go to the ex for the truth, he'll say that he wasn't using you and you might end up going out with him again and the cycle resumes. You go to the cousin, he might lie again (maybe on behalf of your ex). So go to them both and ask about the ordeal. Then just go with your gut. If he was really using you for "items" then why did he dump you? He could have just stayed with you and continued using you.
Hope I helped out, eh? smile