You can buy LEGO in bulk at any official LEGO store. It is called the PAB (Pick-A-Brick) Wall. They will sell them to you buy the cup, or if you need really large quantities, you can also ask them for full boxes. Keep in mind that the selection changes seasonally, and you will not be the only seller who buys parts this way, so there will be competition.
Trading in LEGO isn't an easy way to make a lot of money. The margins are much smaller than they were, and you'll face stiff competition. That said, it is possible to make a profit if you do your research and you're willing to put the effort in. Few of us make a fortune, but many of us make enough to fund the odd purchase for our own collections.
To trade ...
There is a website that tracks price changes on Amazon, Bricklink and E-bay in order to try to compile an index. It has an interesting page called top retired sets, ranking sets by gain compared to initial purchase price.
At the time of this writing it lists 6166-1 (a big tub of LEGO) as number one with 65% gain, and 7200-1 (final duel) as number two with ...
The LEGO aftermarket is changing all the time and there are no defined rules on how a set will appreciate. Over at http://www.brickpicker.com you will find an entire website (plus forum) dedicated to investing in LEGO sets.
The post-EOL rise in price of a set is driven by many factors, some of which are:
If a set is only available for a ...
Most of these sets will have very little potential value. In regards to the parts, most of these parts are constantly being produced and very easily available. There might be some exceptions for Duplo pieces which I am less familiar with.
The modern day 2021 equivalent of these sets would be the 'Classic' range of sets which tend to include the same parts ...
There are now loads of sets which could be considered to be the most valuable. Here is a list of my choices
10179 UCS Millenium Falcon
10182 Cafe Corner
10196 Grand Carousel
10184 Town Plan
Chrome Gold C3PO (only 100 made)
10189 Taj Mahal
5004590 Bat-Pod (only 1000 worldwide and will surely increase in value tremendously)
This list is only my ...