The title here "Rare China Dragon Stamps to be Sold on Apr Auction in Hong Kong " is not correct, it should be titled: "Rare China Dragon Stamps to be for sale on Apr Auction in Hong Kong", they are different meanings.
If we say something is to be sold in Auction, only in one situation, the seller is very, very keen to sell, then he dosn't set a reserve price on the auction or set a very low reserve, this means he has to sell it, and everyboday know that this item will be abosolutely sold out at this price. And then auctioneer will advise it in this way " this one must be sold!!!" to attract people, here "to be sold "is much more confirmed than "must be sold" because "the item "must be sold" maybe is not sold , but "to be sold " means it is going.
Usually we don't like to say " to be sold" before the thing is sold, we like to say it is for sale or it is on market, but" To be sold " is not incorrect.
Rare China Dragon Stamps to be Sold on Apr Auction in Hong Kong means:这些罕见的大龙票将在香港4月的拍卖上被卖掉。Rare China Dragon Stamps to be for sale on Apr Auction in Hong Kong means: 这些罕见的大龙票将于4月在香港拍卖。