I know in the steam sales and occasionally when a good deal pops up people split packs(makes sense we all save a little).
If it gets really common practice to split packs then eventually someone will get burned either in buying a pack and not getting payed or people backing out or whatever.
I personally think sales forums as it is a sale/deal/agreement between people.
The other part is steam key sales. I tried a few giveaways recently but people are just too lazy tbh so most get barely 5 entrys. I have like 4 keys left and want to sell them(even if just token 50p) but again no idea on that.
So more moderation/guidelines for game key involvement please.