A. It was nice seeing you again.

B. It was nice to see you again.

Hello,

1. Which one is correct above?

2. If both are correct, what is the difference?

3. Which one is appropriate to say at the end of a conversation?

Please advise. Thank you in advance.
1. Which one is correct above?-- Both are correct.

2. If both are correct, what is the difference?-- No real difference in intent; it is two ways of saying the same thing.

3. Which one is appropriate to say at the end of a conversation?-- Both are fine in that situation.

Thank you in advance.-- Why do you say this? Are you not planning to take the time to thank us afterward? It sounds a little presumptuous.
Thank you, Mister Micawber, for your helpful answers.

Mister MicawberThank you in advance.-- Why do you say this? Are you not planning to take the time to thank us afterward? It sounds a little presumptuous.
Personally, I make sure I thank someone whom has done me a favour even if I have said "thank you in advance" beforehand. I thought it sounded polite like phrases such as "Your response would be appreciated." I wasn't aware that it sounds a little presumptuous. I would avoid it then.

Thank you for the additional note.
Students: We have free audio pronunciation exercises.
Nothing personal. It is a common tactical error that I am trying to correct on the internet. The phrase makes it sound like the writer is thanking now so that he won't have to thank later!

Both are perfectly ok.

To see is the action verb, the ability to look at something with your own eyes. "Seeing" in that context means "to meet" or "meeting" sb.