Hi jjb1981,
I see three vintage necklaces of akoya pearls. Pearl grades are not standardized so I'm just going by what I see in the photos.
Of the three, the first necklace looks the best to me. The pearls are rounder (but not completely round) and the surface seems clean (not a lot of blemishes). The pearls do not seem to be yellowed with age, unlike the other two necklaces. The luster is not sharp but they are also not dull. It looks like it needs a bath, though-- I see something stuck to the 7th pearl left of center in the inner strand-- and despite the seller's comments about the knots, some of the front knots are dirty. I would clean and restring it. Factor the cost of that into the price you pay; it could cost you $100- to $150 to restring, depending on where you have it done. $800 is not outrageous for that necklace.
The second necklace has semi round pearls that are a bit yellowed, and the 3rd necklace is quite baroque, and quite yellowed, both of which reduce the value. Personally I don't think they are worth what is being quoted.
All that being said, do you have any reason for thinking that your wife would prefer an older, graduated longish, somewhat cream colored and not-fully-round pearl necklace rather then something new / white/ shorter/ non-graduated / round?
Pearls are soft gems and pre-worn strands can become dulled and the nacre become thinner from exposure to skin oils, sweat, cosmetics, pollution etc. If you really want to buy a pre-owned strand, I think the seller should agree to refund if you decide they do not meet your standards when you receive them and get a chance to examine them in person.
Moi, I'd rather receive a new strand of pearls from my husband-- but I buy my own!
You can buy new pearls online for much less than in brick and mortar stores.