Selling used/refurbished – first sale doctrine …

Hi I have an item listed that is used/refurbished. It’s actually listed on Walmart marketplace, but doesn’t really matter in this case. I have sold a few of these Sony products and I keep getting an email from i20 retail basically asking me to c&d and to provide them with info on where I have purchased the items. They are also saying my Sony item is materially different than what authorized sellers can provide. The listing clearly states it’s refurbished and I don’t do anything to them to change them in any way and all accessories are Sony brand. I’m just curious if I am indeed doing something wrong here? Obviously I’m not an authorized Sony reseller, but I was under the impression that it only was necessary if I was selling “new” items. I just got my “final warning” so it had me thinking. Initially I was going to ignore because the items are used, but wanted to see if anyone had any input. I not going to pay a lawyer $3000 to ask them that question, because I’ve been down this road before with a different brand and in the end it seemed like much ado about nothing. They did draft a response letter and sent that, and told me to continue selling and ultimately nothing came of it.

Thanks for any help!