Generating a vanity address is pretty easy - fire up a small tool, define what you're looking for, start working, and wait.
This process is kind repeated thousands of times daily (I'm guessing) and someone somewhere is just ignoring the address generated that would satisfy your search.
Assuming we create a network of a list of "wanted" vanity addresses, and every time someone searching for an address, it will also check the global list, and we can find some way to pay each other for found addresses.
Assuming all of the above - how would I know that the vanity address that you've sold me is "safe"? You created it, you have or at least had the private key with you at some point, you could simply wait and check if the address begins to have some coins in it, and when it has enough coins, you simply use your private key that you sold to me. Problem, right?
Any way around it?