condo purchase _ flip tax responsible by buyer?

It is irrelevant. The only thing that matters to you is the final dollars out of pocket. You adjust your bid price accordingly. The good news is that you are aware of this before you enter negotiations.

I would like to bid based on past sale record.

but, it is unclear for me if the recorded sale prices in the building were based on the assumption that all other previous buyers paid the flip tax.

If the flip tax is negotiable (could be paid by buyer or seller, regardless of the building rule), the selling prices may not be meaningful.

Flip taxes have to be paid in the end. The coop/condo itself doesn't care WHO pays, as long as they get their $'s. So it's always 'negotiable' - the buyer can pay, the seller can pay, they agree it can split it in some way. In the end though, I probably have to agree with patient09 - the only thing that matters to you is the final dollars out of pocket.

If you haven't communicated a bid, why not just lower what you've been thinking by 3% (or 5%) and see how it goes.