Weird stain on Nike Tennis Classic ?

I ordered a pair of Nike Tennis Classic's from Nike.com about 8 years ago. They have been in storage in my closet ever since and have never been worn. I recently pulled them out and noticed they have yellowed somewhat over the years. Also upon looking inside I noticed there is a weird stain on the inside top right portion of the right foot. Would anyone be able to tell me what this is or what could have caused it? Is it related to the "yellowing effect" or is it glue that is showing through? I tried cleaning it off with all kinds of cleaner and nothing seems to help. I was planning on selling these but after finding this stain I doubt anyone would want them. Any advice?
 

Very easy......this stain looks like old blood, so you put them on ebay as game-worn shoes.......you find out which tennis stars 8 years ago wore that size...."Game-worn nike tennis shoes,  with  roger federer blood..........