I need some advice if I buy expired domains with high PR and ranking to increase traffic for my main site?
It's effective or not in this case? if not, why I should not buy.
A direct 301 redirect is telling Google that the old site no longer exists and the new site is at the URL you have specified. In an ideal world all the back links should then be counted at your site.
Yes, buying expire domain is great idea to get rank. Actually, I am not going to mention redirect the those traffic to your domain but expired domain is great for link building strategies.