Buying subs for my car?

[ace]

My friend wants to sell me 1 eight inch sub and an amp for $150 dollars, he says their really nice. He says ill have to buy my own electrical kit for 30$ and pay 80$ to a shop to install it. Is it a good deal? If they don't work or if I'm getting ripped off, is there a way that I'd be able to get my money back? thanks