probably a bug then. I made this mod for myself when dawnguard came out and decided to share with workshop. But now there are much better and refined mods that do what my mod do but better.

try this one instead http://skyrim.nexusmods.com/mods/25790

i have a lvl 15 dark elf though. maybe you need to be higher leveled than that

IIRC you can purchase the basic crossbow and the dwarven crossbow. The items are levelled though so I think you need to about lvl 7 -15 for them to start appearing

I can't find any vendors who sell crossbows but they do sell bolts. crossbows are still craftable however. Is this a bug or does it take time for the vendors to add the crossbows to their inventory?

Yes it does. I should add that to the description.