How do you reply to sellers whose homes you don't want to buy?

[Matt B.]

How do you reply to sellers, whose homes you don't want to buy? The ones that either respond to your website, or call and leave their property information, on your voicemail. 

Do you always call everyone back? What answer do you typically give, when they want to know why you don't want to buy their property?

Looking for a better reply than "Sorry Mr and Mrs Seller,  your house needs $50k in repairs, but it is only worth $40k all fixed up." Don't want to waste a lot of time with these, but at the same time would like to give some sort of response.

Thanks,