Lord Liaden This is kind of difficult to answer. The differences between those old books and the current HERO System are pretty substantial, so updating them for 5E requires more work than the 4E supplements. Most of the material from Escape From Stronghold, as well as many of the old Enemies villains, were updated to 4E and published in Classic Enemies, while the classic "Viper's Nest" adventure was expanded in the 4E VIPER sourcebook. Those books tend to be fairly available on eBay and from used game dealers. OTOH the fact that those books are so dated tends to reduce their resale value in my experience, so it may not be worth your effort to sell them.
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