This is one of three issues that drive me crazy. I am extremely sensitive to the smell of cigarette smoke and nothing makes me want a card less than having it show up smelling like smoke. To me, it is a 100% valid reason to return a card or cards.
I also can't stand musty smelling/feeling cards -- the ones that have been in a monster box sitting in a damp garage or basement or storage locker for years. I can pick these smells up the second I open a package.
The final thing is pet hair. It bothers me to no end when I get a package and there is pet hair stuck to the packing tape and anything taped inside. I have received cards where the pet hair is inside the top loader and penny sleeve.
The last thing I do before I pack any trade/sale up is make sure that my desk area is clean and that I have washed my hands. I send cards out exactly how I would want to receive them. I have posted it here before, I assume that every card I send out is an integral part of a set the other person is building or is a card they have been looking for for their PC forever and finally found it.
I have a list on my desk of people that I will never trade with again because of this type of issue, let alone the list of people who pack cards unsafely or send knowingly damaged cards.