No you don't have to buy it again. You can ask in the MP Customer Support forum for a download reset :)

http://market.renderosity.com/mod/forumpro/showforum.php?forum_id=12374

Once you buy something you don't ever have to rebuy it again to download it if you lose the file. Just ask for a reset if you've run out of downloads.  There is no cost to having that done either :)

The same goes for any of the stores that you buy something from, at least Daz, RDNA, Poser Pros, 3D Commune does. I'm sure the smaller stores do as well.

When you ask for a reset, sometimes it takes a bit of time to get it reset because it is done manually, so if you ask in the middle of the night, chances are people are still sleeping and it won't be reset until the morning.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi