The DIS's counter-argument would be that they are, as Carol explained above, simply providing a marketplace. They're allowing classified ads in much the same way a newspaper does. They are not a party to the transaction.
Now, obviously, if someone sued they would attack that argument, trying to draw a distincition between the paid and unpaid listings. And that could get interesting...possibly even hilarious, if they approached it right. I don't know if a plaintiff could prevail, but they sure could have some fun.
Realistically, however, nobody's likely to sue anyone over a
DVC rental. It has nothing to do with the facts or law -- the transactions are simply not large enough to merit a lawsuit. It would cost more to sue than you could hope to collect.