I'm not convinced that the 'endowment effect' would be an inaccurate heuristic. (Is this being claimed? I'm not sure.)

I don't think it's being claimed that it's a universally bad heuristic (or valuation bias or whatever), but rather that it sometimes leads to bad decisions. For example, I've read speculation* to the effect that it can make renegotiation of contracts trickier (people unwilling to go down), make it harder for e.g. mobile spectrum licenses to end up with the right companies, and (elsewhere) that it may account for people hoarding stuff even when it's extremely unlikely that they will ever have a use for it.

*upon re-skimming, that article also makes reference to an MRI study that purports to find the endowment effect in the brain, and contains a bunch of evpsych speculation about possible evolutionary causes by people with PhDs.