No, it is very unlikely that there was an assumed convention, understanding etc. that slaves should be sold further north for whatever reason.
All though there may be documented cases of individuals that had such thoughts, there is more reliable documentation showing that the opposite is more likely to be true.
As is other cases, proving something that does not exist is almost impossible.
However when such questions are properly analysed (i.e. split into separate aspects), some of which can be proven, one can come to a likely conclusion.
Is it likely that a slave owner would deliberately consider selling their slave to a northern area with the intended purpose of assisting the slave to escape?
- no, more likely they would have considered this a pursuance of fraud
The Wikipedia chapter Robert E. Lee and the Custis slaves show many different aspects of what people thought of the time.
The summary, due to the historical figure of Robert E. Lee, can be considered extensively researched and possibly reflected the general opinion of the times.
There is no hint of the idea of selling slaves further north to assist them in escaping.
On the contrary, the selling of a difficult, rebellious slave further south was probably considered the best solution for most.
How was a slave sold?
Other than the direct selling between owners (in most cases a local solution), Auctions would be, most probably, the norm.
A description of a Slave Auction, 1859 also contradicts the idea that selling a slave to the furthest northern buyer was the last thought on their mind.
Despite the fact that the image shown is from 1829, the conditions described by the New York City reporter in 1859 conforms to the generally accepted way such auctions were held.
Reliable documentation on how slave owners perceived slaves and how they sold slaves at the time contradicts the notion:
that slave owners were supposed to sell their slaves to a more northern plantation.