And this was excellent advice, at least in the days before third party grading. But with the advent of PCGS and NGC, the cost of purchasing upper grade coins soared in relation to their lesser-graded counterparts.
It is common for a rare date Liberty Eagle in AU condition to sell for three times the price of the same coin in EF condition. It is common for an MS63 to sell for twice that of an MS62, and for MS65 coins (for those few dates in which MS65 is available) to sell for more than twice the cost of an MS64.
Of course, it is unwise to pay a much higher cost for a slightly higher grade. To do so means that you will have less money for other dates, and there is a large risk that supply-demand patterns will change in the future, wiping out the premium in the higher grade. This has happened many times for many dates in the years since certification, and will certainly happen again.
I don't recommend buying in grades below VF30. A gold coin is much more attractive with some remaining luster and good details in Liberty's hair and the eagle's feathers. Below VF30, the luster is gone almost completely, and many fine details are lost. Also, the fields begin to look like spring plowing has begun.
I recommend only buying those coins certified by PCGS, NGC or ANACS. Even if you know how to grade coins, there are many potential hazards when buying raw coins: counterfeits, ex-jewelry, polished, altered, saltwater-damage. Also, even if a coin should make a certain grade does not mean it will be easy to get it into a slab with that grade. And a raw gold coin is susceptible to damage, even by the personnel of the grading service.
Only buy one coin of each variety. Avoid the temptation to upgrade.
Sources. Auctions can be an excellent source. Teletrade, Heritage Bullet and Signature sales, Bowers & Merena and Superior auctions all typically have Liberty Eagles in their sales. Stack's tends to auction raw coins, which should be avoided unless you are a professional. There are many dealers, particularly those that advertise in Coin World, that sell Liberty Eagles. Be aware of prices, as sometimes a dealer will offer a coin for up to double what it would sell for at auction. Also, I don't recommend letting a favorite dealer find your missing dates to sell to you, as for this service a 20% premium is often charged.
Selling. Before selling, it may be wise to ship your slabs to NGC or another third party service, to have them regraded. Do not remove the coins from the slab! As graders and grading standards may have changed since the coin was first slabbed, some of the coins may upgrade. After your coins have been returned, your best bet may be to get offers for the entire collection from at least three major dealers. Auctioning a collection is commonly done, but you would be surprised how poorly an unreserved collection can perform at auction, especially after the buyers and sellers fees have been assessed.