Technically, Blood of Gladiators has been out of print for a while. Even Fields of Honor has been out of print for a while. UDE makes one large print run based on expectations of how much they'll need. The only time they've ever gone back to print (as far as has been publicized) was for Heroes of Azeroth.
What your store's owner actually means (even if he/she doesn't realize it) is that the product is no longer available from his/her distributors. This usually happens much quicker with the sets that have a mount loot due to speculators, but eventually happens with every set (except, seemingly, for Dark Portal).
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