'74 And '76 US Mint Sets

I've been putting together a complete set of clad US mint sets over the last year or so buying off ebay. I still have yet to win the '65, '67, '74, '76 and '04 and '05 sets, but otherwise I'm complete back to 1964. I'm being very patient and losing lots of auctions because I want to get them for what I want to pay, not what I'm forced to pay. I've gotten most of them for very good deals compared to numismedia and Red Book prices. It's like the thrill of the hunt type thing. Anyway, I can understand why the '65 and '67 don't go for cheap because of the silver half, and the '04 and '05 sets are a little more expensive because of the extra quarters, but why are the '74 and '76 mint sets going for more than book value on ebay? Can anyone shed some light on this? They're both clad sets with $3.82 face value inside, so why are people paying so much for these on ebay? Like more than numismedia or Red Book prices.