Learn how to keep them correctly, they will reproduce, as long as they are a pair and they get along, and all basic needs are met.
The problem is this, why would anyone sell a reproductive pair, a "proven pair" so quickly? Id venture to say that "proven" means they want to sell them faster because they had this "supposed" pair for so long with no reproduction occuring.
If they are a pair, they might be worth it if they were say between 1-2 years old. They can live a long time, they can reproduce throughout their lives, but if you are looking for monitors of any species, to breed, you should raise them from hatchlings, the most fun part and the best way to learn to keep them correctly.
Of course they grow so fast..





The pictures from hatching on were all taken in around 1 months time, Ill have to get some new ones. They grow very very fast.