Season Seat Member. That is what the Sens marketing seems to use, not that it matters - we're both talking about the same thing.
A pair of first round 300 level tickets is about $500 right now on Stubhub, without Sens/Leafs even being confirmed. It wouldn't shock me if they shot up if/when it is confirmed. It's not just the Leafs, it's the Sens not having playoffs in 10 years, so there is demand both ways.
I am not going to shame anyone for selling their tickets. Some people just won't be in a position where they can look at 6 tickets that they can possibly sell for 2000+ profit and keep them. I use profit loosely, because anybody doing that who originally intended to go to the games is likely pouring that 2k right back into funding their SSM package. If you have something that has inflated in value and you choose to keep it instead of selling it, that is the same (by proxy) as you choosing to buy it for that value. A lot of people holding these tickets wouldn't be able to responsibly pay $2500 for first round 300 level tickets so they are going to sell them.
With that said, I don't think the arena will be as taken over as it is for regular season games. I think a big chunk of the fanbase is clamouring for playoffs, and while some SSMs are probably ticket brokers, a lot of them are fans. I think the fanbase has learned not to take playoffs for granted and they probably want to go. Where as regular season Leafs games get taken over because there is a substitute product Sens fans can go to for much less money (games against any other opponent), so they sell or don't buy Leafs tickets, and go see Ottawa VS Columbus for 1/5th of the price. I don't think the same substitute product exists for playoffs so there will be more Sens fans. I would guess it will be like 60/40 Sens fans or 75/25. Something like that.