Is buying computer by internet safe?

Yes it is safe if you get it from a reliable online store or seller that has good reviews and reputation. Sites that I have personally purchased products are newegg.com and tigerdirect.com. These sites are safe and ship products fast. I advise that before you make any purchases online that you look at reviews from the online store to make sure that they are reliable sellers.Here is a site you can use to look at reviews on different online stores:http://www.resellerratings.com/rlist-s1-n2.html
Or you can just google the name of the store and add review to the search and you should find some reviews on it.

I don't have any experience with buying computers online. Anyway, follow the steps given below to find if an online website is legit and safe to buy any product from them or not.
1.Use Whois lookup tool like WhoisXY.com to check the whois information. Legit companies properly update whois database.
2.Check whether domain owner contact details and E-mail address is currently active or not.
3.Take some reviews about that company using Scamadviser.com, Webutation.net.
I am sure above the steps would be very useful to check any company is legit or scam.I hope it will be very useful to you.