It is a bit of a weird one.
Do you know if the concession stamps would have likely been ones you used on the padded mailer?
I think you are probably right and the seller (or poster) is recycling the padded mailer. On the balance of probability though, they didn't pick it up from the tip, they either bought from you at an earlier stage or else someone they know did, and has passed on the padded bag to the seller or poster. Again, we have no evidence the items inside were sold, they could have been a gift for all you know.
I doubt you could do an easy search of ebay either by just searching for a sale of those DVDs because I think it is the case that those records only show for a month, you won't see August sales.
Personally, I don't think you have a problem here. They were delivered to you, you have no evidence as to who sent them or who was meant to receive them, there is no 'lost property' spot you can drop them off to, so just keep them or gift to an op shop or someone who would like them.
You say it was originally shipped between late Aug/early Oct. In that case, if it was an ebay purchase, then it is long finished. A buyer could, ages ago, have easily filed for item not received and had a refund.
There is actually nothing though to suggest it contained an ebay purchase or was being sent to an ebay buyer. All you really know is that a padded bag you once sent an ebay item out in has been reused.
Just consider this as one of life's little stumbles for the sender. Not everything can be fixed.
