Does the auction say what the stain actually is? Coach nylon typically cleans up wonderfully but you should probably find out what the mark is first. According to Coach's website, nylon bags should be cleaned with mild soap and water but I am sure you can also use Shout wipes or baby wipes, just test a small inconspicuous area first.
Have you emailed the seller to ask? I would! If you don't get a response you might want to pass on it. Worst case scenario is that it's ink and that might never come out.
ETA: Okay, I just found the auction. There's a good chance that you might not get a response from the seller seeing the end is so close and because they are high volume sellers. The price is still quite low so I guess you have to ask yourself if it's worth risking the money on it.