How to buy a lot of LEGO Technic bricks?
Usually the most economical way to acquire LEGO elements is to just buy full sets (ideally on sale). So if you have a particular design in mind, look at the currently available - or recently retired - LEGO Technic sets and see which one would have all or most of the pieces you would like to work with.

You can also check eBay and BrickLink for used LEGO Technic sets. They are less expensive than new condition sets, but are still well taken care of by the original owner. If a few pieces are missing they are even cheaper. Building up a LEGO Technic collection this way usually works out better than just buying random lots from resellers. 

In general BrickLink is the better way to go as the sellers are also dedicated LEGO fans and won't sell you something they wouldn't be happy with themselves. However eBay could be cheaper. If there are good pictures of the used set, and they are from the original owner, you can find great deals.

In summary, when you buy pre-owned, it is best to find slightly used full sets from original adult owners. The pieces will be in better condition and the selection of parts will also be better, rather than just buying random lots where you don't know the history and condition of the pieces, and the seller is not the original owner.