Why do you think Valve would allow people to trade if they wouldn't think it's safe? The probation tag will disappear after a while. They might have done something wrong in the past and Valve temporarily trade banned them for that but it's in their best interest not to repeat the same mistake again. Nonetheless you should be more cautious when trading with someone on probation and check if their items aren't hijacked or scammed.

No, but you can get marked by steamrep if you trade with a marked steamrep scammer repeatedly and knowingly.

Someone who's on probation isn't automatically marked as a scammer on Steamrep. Of course it's advised to check their Steamrep profile and see what offense they might have commited. You should avoid scammers at all cost but if someone is on probation it doesn't necesserily mean they are going to scam you or that they've commited large crimes in the past (because chances are Valve wouldn't allow them to trade then).