To talk about (something or someone) in such a way as to make the thing or person sound better than it actually is
If someone talks up a particular thing, they make it sound more interesting, valuable, or likely than it originally seemed. Politicians accuse the media of talking up the possibility of a riot
To talk someone or something up in negotiations means to persuade someone to pay more money than they originally offered or wanted to. Clarke kept talking the price up, while Wilkinson kept knocking it down