Came across a nice deal on a set of SM60/BF10 in astromart, upon some communication when I'm about to make the deal, I asked a couple more photographs and found that the setup indeed is only a BF5 but not a BF10. I've also asked about the age of the filter set, and found that the seller is not the original owner but he's so nice to provide the contact of the original owner.
The original owner told me that the filter is over 4 years old, not a very good sign?
And then since it's not a BF10, the seller offered to sell without the BF5, so I deduct the price of a BF10 with some discount, and offered again. The seller refused.
It shows the importance of effective communication. Actually, I'd be in a very bad situation to buy such an expensive filter and later found that it's indeed less than the posted specification and much older than my expectations.