Do they have the right to do that??

I went into my local computer shop a few hours ago, its nothing big. I was looking for a new graphics card for the computer i have just bought. The guy that owns it asked me where i got the computer and as soon as i said Dell he refused to sell me anything as i had not bought a computer from him abd that he wanted me to leave the shop now. I left without a fuss but can someone actually do that? just because i did not buy from him in the first place?