I purchased a phone in offer up. I have been using it for a bit and had no issues. I changed carriers only to find the seller had put a device in the phone to mask that it was blocked by another carrier. Offerup deletes all messages for some reason when I switched phones and logged in so I can't access the seller to contact him or report him. Likewise offerup has no email or phone number and isn't answering the strange twitter account. How do I gain access to the logged messages on my account to flag and or deal with this seller?
Your messages are on the AWS servers and "switching" phones does not automatically delete messages on any app.
If you login via OfferUp.com, you should be able to view the messages and the seller's profile in your "My offers" tab, under "Buying".