thank you for your comment @angelfish. Actually, comments can still be added to expired posts, and those comments can still be upvoted. That's also the basis of my "Post Vote Slider and Vote Past Payout" script"!
Regarding the internal market, there are no additional fees levied for exchanging STEEM<->SBD, though each order does use up a bit of your allocated "bandwidth" (just as any vote or other transaction does on STEEM blockchain).