We've seen it all means "We HAVE seen it all" so, here is used the preset perfect tense. We use this tense for these situations: putting emphasis on the result action that is still going on action that stopped recently finished action that has an influence on the present action that has taken place once, never or several times before the moment of speaking Signal words: already, ever, just, never, not yet, so far, till now, up to now I hope this helps. If you have any further questions, feel free to ask me.

'We have seen it all' is quite literal - meaning we have seen everything. Although it is obviously hyperbole, it is usually used to suggest that the speaker is very experienced, and has dealt with a wide range of situations.