I've used flickr in the past, and will probably play around with it a bit again next year. Most of the 'use' requests that come in from (Ironically) "buyers" who don't have a budget. - How do you buy something with no money? Oh, wait, they bait you with fame & fortune by offering you the 'gift' & 'opportunity' of a credit line.
That said, I've had a couple legitimate sales come through on flickr. I've also caught someone that stole and used one of my flickr shots without my permission. I wound up billing (& getting) four figures for what would have been a three figure sale had they just asked first.
If you want to use flickr as a sales channel, there are buyers out there that use it, but they're usually looking in cheap or free mode.
HTH,