It's really a personal decision based on your financial circumstances.
The offer seems really low to me for an offer from an end user. I sold AcaiBerry.co.uk last year for around £1000 and I think there are end users out there who in time would be willing to pay more than $200 for your domain.
However, it depends on your financial circumstances and business model. If you could use the money, or perhaps reinvest it in something profitably, it may make sense to sell. Having cash on hand is always nice. Plus, I doubt you could sell this domain on the forums and get that much money.
Another thing to keep in mind is that Acai Berry is very trendy right now. It may go out of fashion, which would decrease the value of the domain.
Regardless of what you decide, I'd try to negotiate a higher price. If you don't ask for it, you won't get it.
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