Complaint sub-issue: Credit card company isn't resolving a dispute about a purchase on your statement

On XX/XX/XXXX I ordered a cylinder head truck engine part from XXXX ( Merchant/Seller ) for {$370.00} charged to my Citibank credit card. Merchant 's website advertised the part as completely remanufactured in like new condition by " extremely meticulous quality control process and highest standards '' covered by a 3 year warranty. Merchant did not warn consumers that buyer assumes the risk of defective or nonconforming goods or the cost of shipping or return shipping in the event the goods are defective or nonconforming. On XX/XX/XXXX, I received the engine part by XXXX  shipping. On XX/XX/XXXX, I inspected the part and discovered it was defective and nonconforming because it was partly remanufactured and still dirty with carbon buildup, oil, rust and damaged components. I tried in good faith to resolve the matter with the Merchant several times, but Merchant failed to act. Therefore, on XX/XX/XXXX, I reshipped the defective engine part back to the Merchant using Merchant 's XXXX   return shipping label included with the original shipment. XXXX confirmed Merchant received the return on XX/XX/XXXX. I waited until XX/XX/XXXX for Merchant to process my refund but Merchant refused to issue credit to my Citibank card. On XX/XX/XXXX, I received my Citibank card statement that showed Merchant charged my card in the amount of {$370.00} on XX/XX/XXXX but there was no corresponding credit for the returned engine part. On XX/XX/XXXX, I called Citibank Customer Service to dispute the charge and requested a chargeback because the engine part was defective and had been returned to Merchant. On a recorded phone call, two Citibank dispute experts, XXXX and XXXX, said they would promptly investigate the matter to issue a credit and send me a reply by mail in a couple of days. However, they never replied, so on XX/XX/XXXX, I wrote Citibank a letter to the address specified for disputes ( XXXX  XXXX XXXX XXXX, SD XXXX ) with written request that Citibank chargeback the defective item that I returned to Merchant in XXXX and credit my account for {$370.00}. On XX/XX/XXXX, Citibank called my work to collect the disputed balance of {$370.00}. After several phone transfers, I spoke with XXXX with Citibank Billing Department on a recorded phone call and told her the purchase has been disputed several times with no response from Citibank. XXXX said that Citibank received my written dispute letter on XX/XX/XXXX, that they were still investigating and that they will reply soon by mail. However, to date Citibank has failed to reply to either of my disputes or to issue a credit to my card account even though the defective item was returned to Merchant eight days after I received it. Instead, Citibank has damaged my credit rating by reporting my card account as delinquent to credit bureaus, continuously harasses me with calls to my work to demand payment and has failed to protect my consumer rights for defective goods purchased with a credit card. All the while Citibank has protected the fraudulent Merchant that has received a windfall by having both the item returned to them and keeping the credit card proceeds from the sale!