Legal to sell them? Yes.

Legal for someone to use the resulting ticket? At least in some jurisdictions, no in the strict sense of the word "illegal," since doing so would be in violation of the ticket's terms and conditions.

From a practical point of view, anyone who has just 120K miles and hasn't done this on a regular basis is at the mercy of the airline folks who look for this sort of thing. They vary in aggressiveness and in whether or not they have bigger fish to fry at a given moment, but if they want to catch someone they have tools with which to do so. If they get suspicious - especially if the purchaser can't answer some basic questions about you - the likely result is closing your account, which includes losing any miles still in it, and invalidating the purchased ticket, forcing its purchaser to pay the walk-up fare or not fly. If they catch it on the return, not flying may not be an option.

I've given award tickets to family members. None has ever been questioned about how they got their ticket. Their connection to me was generally not obvious from any information the airline had. The airline would have no way of knowing if one of them had given me a $300 check the week before. So, I'd say your chances of getting away with it once are pretty good - but there is a risk. The more often you do it, the more likely the airline is to spot it and take action.