or you know... you can just play with my dropmod on.
http://dropcanvas.com/jx5wfScreenshots
http://imgur.com/G17L3pr &
http://imgur.com/KCIAIkD... but each to his own xD
You're right in that not all chests has a chance to drop legendaries. 0% chance for most. Legendaries are only in a few loot tables, the most common are likely: RewardBig; MegaBoss; MiniBoss; etc. It is not until later in the game that they started making common monsters drop legendaries, for example: Death knights drop from the T_ST_RingOrAmuletLegendary loot table with a frequency of 2 (2%). (check treasuretable.xlsm after unpacking files for more info - and check things loot tables in the editor)
If you want to buy legendaries, I recommend the skeleton merchant outside Hunter's edge, which has a very sweet loot table (get attitude to 100, bartering 5 and it is cheap). His table includes T_ST_WeaponLegendary;T_ST_ArmorLegendary and T_ST_SecGearLegendary - which means he can sell a very, very large variety of legendaries.
Edit: guess I should have included some kind of spoiler tag? Just noticed all your videos are from the Cyseal area. Oh well!^^