They say a Vodacom sim must have been used in the device within the last 90 days, so doesn't have to be exclusively Vodacom sims.
From my experience as long as the handset is under warranty they'll fix it, I've taken a previously jail broken device into Vodacom and they didn't bat an eye lid. They have never requested pop, they use the serial number to check the warranty status.
As has been mentioned with a contract phone the real risk is the seller not paying the contract and handset then being blacklisted, but from my experience that's not very common, though it's better if you know the person you are buying the phone from or know of them to be reputable.
Do make a sale agreement though, with wording to the effect that the seller is saying the phone belongs to them, if it gets blacklisted due to non payment then you open a case of fraud.
Also consider the risk, buying a contract phone for over R4k from an unknown person is taking a real chance, R6-10k is starting to really get very dodgy, a blacklisted phone is essentially permanently bricked... Some people do it though, of you can afford the risk I suppose then why not.