Regarding the myth of surplus LTZ1000/A parts, according to Linear Tech, there are no surplus parts or overstocks, every one of them are accounted for, including the ones sold through their online shop. LT keeps close track of the LTZs that they sell to the OEMs, they trade data back and forth, the OEMS do not, I repeat, do not have surplus or overstock to sell to anybody, they buy just what they need and nothing more.
Unfortunately, the condition of used parts removed from equipment is available before purchase, they may be in good shape but odds are they are not, de-soldering can cause changes or damage to the chip, therefore, if you are trying to achieve spec in an Vref, buy the LTZ1000/A from a reputable source and forget about the sellers on eBay or elsewhere, they are selling you a pig-in-a-poke.