I was speaking to a fellow PhD researcher today who said they had put a seminar given as part of an academic interview process on their CV.

I've never considered doing this as I thought is was 'not the done thing' and is somewhat cheeky, but the idea does seem appealing for padding out one's CV and particularly if the presentation/seminar was given at a prestigious institution.

Is this common practice or am I right to think it's wrong?

I wouldn't, only conference/large meeting presentations where people external to your uni would be present.

Seems a little desperate?

^^ that's what I was thinking, although it is tempting.