I used to work at an Eckerd Drug Store, and after the first time I had to sell a package, I gave the matter some very serious reflection. What I finally concluded was this:
There is nothing intrinsically evil about a condom. Its physical makeup is no different from a balloon or glove. Purchasing a condom is not intrinsically evil. The person could be purchasing them for entirely non-sexual reasons. Ultimately, refusing to sell a package of condoms comes down to an act of judgment upon those who are buying them. For me decide not to sell them because “I know what they’re gonna use them for” is to usurp God’s place as Judge of their hearts.
One related thing I read recently about condom use: “The Church is not nearly so tyrannical as to forbid ever wearing condoms. In fact, we may wear a condom at any time – so long as it’s not during sex!”