The background is that afternic has been sending unsolicited offers for a two word domain for over a year. After a 6 month break, they are trying again.
The two words are quite brandable and have a certain ring to them.
I am pretty sure who the buyer is, and it is a household brand. They have built an entire marketing campaign around these two exact words, in the same order. The spokesperson is a well known personality.
Given the above, an offer in the *low* xxxxx seems somewhat like a lowball.
Any words of wisdom on how to proceed? Lay a number on the table? Approach the real buyer?

I think it depends on your personal finance situation, do you need money now or not?

I'd make a counter via afternic. Give them the opportunity to see what you're looking for. That may get a more reasonable counter from them and you may make a deal.
I would not take the chance of approaching whomever you think the real buyer is. If it's not them, and they can claim any sort of trademark -- bona fide use in commerce may suffice if it's not an actual registered trademark -- you could set yourself up for a UDRP and lose the domain.