+0
"Everyone was excited. The party would be fun."
"We would pay what he asked, but he gave up selling his car."

In these sentences above, can I use "would have+V3" as below:

"Everyone was excited. The party would have been fun."
"We would have paid what he asked, but he gave up selling his car."
+1
"Everyone was excited. The party would be fun." -- At the time they were excited, the party was still in the future.

"Everyone was excited. The party would have been fun." -- Sounds as if the party was cancelled; seems somewhat self-contradictory because presumably people would no longer be excited if that was the case.

In the other sentences, "would have" seems to be the correct choice for what you are trying to say, but "gave up" is not really the right phrase. You can say, for example, "We would have paid what he asked, but he decided not to sell his car after all."
Comments  
I think that "would+v1" should be used in these sentences as "would have+v3" needs an if-clause. For example, for the second sentence, we can say "we would have paid what he asked if he hadn't given up selling his car" in the same meaning (For the first sentence, I think that there is no way to express in "would have+v3" pattern). Thank you.
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 GPY's reply was promoted to an answer.
So, I think that both types can be acceptable because each one seems to give a different meaning as GPY explained for the 1st example. In the same way, I think that the situation is the same for the 2nd sentence, that is, if we say "we would pay ...", it sounds like we won't pay, and if we say "we would have paid ...", it sounds like we didn't pay. Thank you.
Thank you so much. I see better now. It seems that the correct explanation is in the last two replies before this. Thank you.
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