Rational means the same thing it means in other contexts. If a rational man is faced with buying the same product for two prices he will chose the lower as a rational choice. That is an example of seeking his "best interest" as well since it gives a greater value to his money.
The assumption of rationality is the same assumption that democracy builds upon.
However, you must consider that the assumption of both requires that there be complete information available about the alternatives.
How often do you see "the same product [sold] for two prices"?
If I chose to buy from a local grocery because I like family owned stores do I act irrational?
If I buy more expensive product because I do not have time to look, do I act irrational?
If I drive around and waste time and money to buy something useless but what I like, do I act irrational?
If I am motivated by the emotions or impulses and not by calculation, , do I act irrational?
If I give money to a bum on a street do I act irrational?
If I chose to work less in order to have more free time do I act irrational?
If I buy expensive caviar instead of cheap gruel do I act irrational?
If I decide to save money and then die leaving them to the inheritors, do I act irrational?
Is my best interest different from the interests of my close relatives or from my friends, or from my country? Which should I chose?
Is my best interest to enjoy money now or to save them to get better return later?
If I spend money on myself instead on my family, do I act irrational?
If I can steal without being caught or punished, do I act irrational?
If I vote for the government which will serve my private interests by handouts, do I act irrational?
If that were true, than stagers would be out of jobs.