It sounds so simple & attractive to offload (say) $20K of CDX...

It sounds so simple & attractive to offload (say) $20K of CDX today at 7.5c because you know you’re picking up (say) $30K at 5c in the placement.

But... as you know human nature dictates that the majority of punters will be thinking “but what if she runs to 15c next week?” and very few of those placement shares will end up getting flipped. Things like this are normally only an issue when a lead broker has been gifted a bucket load or have placed a few $ mill of it with mates that can & will cut & run.