0 xp is worth more because there's extra BP to gain from them, and 'mint condition' CRs are a lot less common than full xp ones.
Private sale prices are a bit lower on occasion because the buyer is little away from the market price, or the market price doesn't reflect to the real cost of the card for whatever reason.
There are several reasons for these things.
Private sales are often related to "bigger campaigns" for example when someone tries to monopolize a card. Big lot could be sold either underprice to person attempting the takeover because both parties knows card isnt really worth that much unless the takeover is successfull and it might be upto that lot to be successfull, so its kind of "share the profit" situation and other party bears the risk of investment thus he gets good discount and other party gets quick cash for whole lot.
Also sometimes price gets high quick and owner of lot doesnt want to push the market price down by forcing sales, so he asks for wholedeal on forums instead of messing the new market price with his cards.
In other times there might be overprice because lot-owner knows his lot must be bought in order to "own" that cards supply, thus he asks openly for overprice. It is still possible to sell if the "overprice" is reasonable - if you are planning to put 1000 market price card to 2000+ you can pay 1500 and still get profits for those "overpriced" cards if your steering succeeds, but if that lot stays on the market it would block your sales so its better to get that lot away before it blocks you and lot owner becomes real competitor in sales. In these cases you pay the owner overprice so he lets his cards go "early" and loses potential (and uncertain) big profits for guaranteed decent profits.
So that goes mainly for a whole lot of cards, not less than 20 or so at least?
That quantity varies from card to card. At one point in time, I own 400 Leviatonn 0 xp and that is considered quite a lot considering not many own such a big amount of card and considering he is out of NB for so long and the supply for 0 xp is quite low considering not many people own huge amounts of it. I know someone with 1k+ Leviatonn 0 xp personally. Now that is a low.
If someone owns 400 Shazam 0 xp as compared to 400 Leviatonn 0 xp, that Shazam lot might not be considered as a "bigger quantity" because someone can easily own 1k+ of him and his price is cheap and on top of that, he is common and still in new blood.
Deepend and Thoazol basically sum it all up. Just adding some additional input.