Complaint Review: ABC Gift Cards

I sold some completely valid and full gift cards to ABC Gift Cards over 3 months ago. I was contacted on April 24th requesting further information on those gift cards and I told the representative exactly how I had obtained them and that I had entered the full card number on their site to verify the amounts before I sent them in (guaranteeing that the cards were valid and had the funds on them).  ABC Gift Cards claimed they had attemped to call me several times, which is not true. I do not have any record of missing any calls from them on or before April 24th. I did not receive any further correspondence from ABC Gift Cards until I received a debt collection letter on 5/21. I believe that they are wrongly attempting to collect on a debt that is not mine. The gift cards I provided to ABC Gift Cards were valid - they were purchased directly through the American Express points program.

I was not given any notification that I was being sent to a collections agency, which I understand is not required by law, but it would have been appreciated - I could have tried to resolve the issue directly with ABC Gift Cards. In addition to this, I was never sent a bill or invoice for the amount - I was never requested to pay the amount of $164.00. Also, I'm not sure why the debt is $164.00 since ABC Gift Cards is claiming two $100 gift cards are invalid. That doesn't make $164.00, that makes $200.00.  It also doesn't make sense that only 2 of the several gift cards I sent were invalid - why not all of them?