I am a collector of "opened" product and am personally not comfortable buying unopened products such as what you have mentioned.
The main reason I feel uncomfortable with unopened material from this age is because of the horror stories you mention and the fact that items such as empty boxes, used wrappers, and larger groupings of NM cards are all readily available. Sellers such as NESC typically offer bulk groups of NM appearing cards, from 75 and other 70's issues also. There are multiple listings for empty 75 Topps wax boxes currently listed on ebay. And here is a recent listing that offered a large group of 75 Topps wax wrappers:
http://www.ebay.com/itm/1975-Topps-B...item4d2247ca29
Combine these readily available items together in certain ways, and you have what appears to be some "unopened" or recently opened product.
You are right to ask the questions you have, so you would do best to further research on how to determine if wax packs have been resealed, apply common sense and the knowledge you have gained to what you have seen, and take time to think this purchase decision through.