Velocity sensitivity is a key feature. The keyboard sends the information of how hard you hit the key to the sequencer. The sequencer relays that to the synth. Often the synth will have the sound play brighter and louder when you hit the key harder, and quieter and duller when you hit it softer. It adds a great deal of expression to your music and you probably shouldn't get a keyboard without it.
I'd advise buying a keyboard at a flea market because you have no idea if any of the keys are broken or if it is velocity sensitive, etc. If it doesn't work, you can't take it back to a flea market...
-=bumpus=-