I'd have no problem with used. I'd prefer to inspect it before I bought it though or have a favorable return policy. The 35 year old Suntour 5 speed I took off my '78 Raleigh is was smoother running and had no play in it unlike the new 7 speed Shimano I replaced it with. My original Campy shifter though made for 5 speeds works very well with the seven speed.

Chain skip can also be a worn out chain. Which chains are fairly cheap, so if yours is not new, then you might want to try that. Though you can measure the stretch, I do not find it easy to get a precise measure with a common tape measure and old eyes. I've been wanting to buy that Park tool for checking stretch, but I keep forgetting (another issue for us vintage types).

I don't think you'd see any skip issues with a worn cog and new chain. Though the teeth are worn, they still are in the correct position to hit the proper position on the chain. And just because it's used doesn't mean it is worn out of spec.