I am a long term season ticket holder with lots of mates who are long term season ticket holders. I know of no one selling away tickets on. They may pass them on to fellow spurs friends and families but selling on for profit is in my view a very rare.
Even that itself is unfair though paddy, those with the chance will apply for every away game regardless of whether they plan to go or not. They get rewarded with ticketing points even if they don't go and pass ticket on at FV to their social network because the demand is there, fans start paying over the odds.