Actually, you can award up to 50 sMerits per person per 30 days, so a single post may receive that amount from a single person. I personally have not (I believe) gone over 4 or 5 sMerits per TX, but I’m a bit mean in terms of awarding amounts per post. Others are more generous and/or pressed on time, thus awarding larger amounts (pretty exceptional though to see large sMerit TXs). On some occasions, larger chunks may discretely be awarded to aid the ranking on a person who is nearly on his next rank.

I doubt that there is much sMerit trading going on, at least not in large amounts. Not that many people manage significant amounts nowadays, and there are always people on the lookout. For example, we can see all large sMerit TXs since the 01/11/2019 here (for >= 20 sMerits):
https://public.tableau.com/shared/XMFYK57G9?:display_count=y&:origin=viz_share_link

Most are from @theymos for the art contest. That is an exceptional event though. There stranger ones are normally trying to hide in places such as Ann threads, but there are less of those as time goes by.