The most common and most likely answer to this crossword clue is the 7 plus 6 letter word CAPTIVE MARKET
I've seen this crossword clue in The Times
C
A
P
T
I
V
E
M
A
R
K
E
T
Buddy explains!
The straight part
We know that "Where slaves are sold at a monopoly price" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where slaves are sold at a monopoly price = captive market.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.