Your initial instinct that this is a civil case appears correct. I would suggest not calling the police. I would further suggest sending in the demand outlining your damages to the at-fault party. Should they be unwilling be to negotiate, I would suggest consulting with a civil litigation lawyer in your jurisdiction. Best of luck.

No police, he has committed no crime, they will do nothing for you. No basis to sue, he has not breached the agreement yet to provide you with a pitbull puppy. Unless you have a contract that states when he was supposed to provide you with pictures and paperwork, then the issue of whether he has breached that part of the contract will depend on what a court finds to be a "reasonable time", which depends on the specific facts and circumstances, what is normal and expected with these types of contracts, what specific representations were made about the time frames for compliance, etc. A "few days" passage of time is never going to be deemed material enough to justify a declaration of breach of contract and right to receive a full refund. Finally, Maryland law does not permit award of damages for psychological or emotional injury arising out of a breach of contract. You can only sue for return of your money. Since you have documented both your and his agreement to terminate the contract and receive a refund of your money, if he has failed to refund you within a "reasonable time" then you can sue him in small claims court for the $300, plus court costs, but no more.