A: I'm an angel. Do you buy it?
B: As real as it gotta be. I will buy it.

I don't understand B's say. Please help me.
Buy it means 'believe it' or 'accept it' in this case. It sounds like a movie quote. To me, B means that the story or the concept is real or honest or true, therefore I will accept it. "He told me a story about why he was late but I didn't buy it." Does that make sense?
Thanks, Maple.

I understand you but see this sentence: I'm going to run as fast as can. Okay, that makes sense - I will run at my best pace.

"As real as it is going to be" doesn't really click in that way. Perhaps, B is saying that she/he doesn't really care how much real it is, she/he would buy it nonetheless.

Are you really going to buy this Seiko watch. I think it's a replica. I don't care so as real as it's going to be I will buy it because I love it.

Please guide me. Thanks.
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If you are going to ask a question, why not give as much information as you can? If you had time to question my answer, then you had time to post more information in your original post. Right? 'A' related to one use of 'buy' and 'B', it seems now, related to another. If you knew the answer, why did you ask? You had lots of time to question my answer but not much time when you posted, it seems. I regret trying to help you.
My sincerest apologies.

I didn't question your answer. We were thinking from different perspectives and perhaps in different mental contexts. When your answer didn't click with me, I worked on why it wasn't making too much sense to me. Then, I came up with own explanation and presented it to you to confirm it. When I asked the question, I didn't have any explanation. Your reply, though didn't solved the confusion completely, it did give me further direction to find the solution.

I'm sorry.