post page 404 error

i purchased a wp site from someone
i successfully setted it up today, and i can able to go to homepage
but any post page that i go to gives me an 404 error
if i go to admin page, i can see those posts, and the urls are the same, and if i click it through admin page i still get an 404 error

can anyone help me with this problem? thx
the website url is http://www.toptrailers.net