[djohn]

I don't want to offend someone who has their card(s) priced way too high.

Maybe it's just me, but I find it more offensive to be asked what my bottom line is than to get a lowball offer.
As for your specific instance, it looks like the seller is asking 4x what the last one sold for; though the last one wasn't graded. The seller also seems to think that a BGS9 is some amazingly high grade for a modern card, and thinks it would cross to a PSA10, even though there is only one 9.5 subgrade. Personally I wouldn't pay much of a premium for a BGS9 versus a raw card from a few years ago, unless there were known condition issues with the set. If I was interested in the card I would simply send a best offer of $400-$500 and see what they counter back with. Hopefully they counter and don't just turn down the offer, as that would piss me off, especially after their message.