Wouldn't we say in English a hundred not a thousand or one hundred etc?
I thought "ad" meant "sell" as well as "give." It rejected my answer, "You're selling one hundred, not one thousand!"
I got an answer to a similar question recently. The proper sell is elad. Ad is sort of rude, or for friends... actually I am not quite sure where the line is, but I guess the safe bet is elad-sell, ad-give. But (meg)vesz can mean take and buy.