My father ... told me that ... in about 6 months to a year from now, there will be such shortage of cash that there will be a lot of sellers wanting to get rid of their old bills to liquidate some cash but there won't be enough buyers to bid the bills up to book value.
As others have commented it all depends on what you consider a "good deal." Its very difficult to get a good deal when its being auctioned on the Internet and you cannot inspect the note personally unless it is being sold by a reputable dealer. Yes- it may be wise to wait 6 months to a year. But then again, it may be that in 6 months to a year things begin to turn around, and the promising momentum gets people spending/investing again. If that is the case, then you could miss out on a lot of good deals while waiting for things to get worse.
Most collectors who've been collecting for any length of time know that there is "book value" and "market value" and in times like these there can be quite a discrepency. Generally speaking-- hard economic times usually mean the market gets a little softer and notes can be purchased below book.
Common notes will loose demand, as supply is abundant - sale price will usually be a lower book value. Customers don’t want to spend, and sellers need to move notes, market is weaker, deals can be had.
Common notes have always been a tough sell and usually only move to newbies in this hobby.
Unfortunately, what we're seeing is too many "Gold Flags" or "UNC" sequential "ink smeared" 1969 $20 with "small stains" on the reverse, and similar chaffe coming out of the woodwork. Sellers are "dressing up" ridiculously common notes and using any angle to sell them.
Rare notes will still remain in demand, as supply is always scarce – sale price will usually remain around the book value.
Despite how gloomy the economy appears to be -- I agree that rare notes will remain in demand. I have seen a few go for less than what I would have expected-- but I think there's so few being offered right now--that when they do go up for bid--they attract more attention than before. I have recently seen low grade 2 digit radars auctioned for above book.
If you want a good deal -- I think the trick is to find a rare note that not all collectors are scrambling to buy. What is in demand often goes in cycles. Be patient (do your homework), inspect the note thoroughly (I usually bid up to a grade lower than what is stated on the Interent), bid only on notes that have been supersized or properly scanned (unless you know this note to be in good shape), and the deals will come to you.