Which marketplaces are you talking about? if the earnings shown come from domains that are measured in the marketplace then yes, the earnings are accurate. eg. Above.com's stats are real since they come straight from the users' performance.
There are two factors that affect the earnings you might get when you transfer a parked domain from one owner to the other:
a. The revenue share of the new owner with the parking service
b. Whether the domain is a typo or not.
if the seller is a veteran in parking they will most probably have a better share than someone that is new in the parking business. So, if you are new then chances are the earnings will be lower
typos depend on the original website's performance. If that website performs well then the typo will also perform well. That can continue as long as the original website performs.
With non-typos, since they depend on dying traffic, the earnings typically get into the unprofitable state within 1 to 3 years.