Really can't tell what that is (dirt or?) but the faster you bring such issues up with the seller, the better your chance of getting satisfaction. It's already been 6 days.
Wheezing & stuck shed aren't good signs either, as far as what you can expect from this seller.And checking out a new snakes is totally on you...doesn't matter
if the snake is cooperative or throwing a fit, it's up to you to determine his health & condition upon arrival. Poor time to ask, but what is the seller's guarantee, if any?