The only dragon I don't have is the bone dragon. I'm not sure of the exact date when I started playing. I didn't think the bone was in the market, but I think it may have been from what i have been reading. When I first stated playing October last year I didn't really know what I was looking for as far as dragons were concerned, and I didn't find this forum until late Jan this year. This has been a great help to me - the tips for breeding etc are second to none, not to mention the people on this forum.
I'm disappointed with myself that I didn't try to breed a second Sakura dragon as I won't have that egg to display on my pedestal. I got two emerald dragons and I hatched both, and I tried as hard as I could to get a third for an egg pedestal, but it didn't happen. If I get another ruby, I think I will display the egg rather than hatch it.
The hibernation cave is so important when it comes to the strategy of the game. The last thing you want to do is sell off dragons that you have leveled up. Also, you will have worked this out by now - always level up hybrid dragons first rather than single element dragons, as each element in one hybrid levels up those respective shrines. Bloom was a great dragon because she covered 3 elements - cold, plant and lightening. Panlong covered the other 4 elements - earth, fire air and water. Therefore I have a lot of each dragon (even hiding out in the hibernation cave). Breeding those 2 dragons covered the entire 7 habitats all at once - too easy, and less money spent on food. I thought I was doing really well, and then the metal habitat was released so I needed to rethink my strategy.
Don't spend money where you don't need to if you can use the hibernation cave and level up hybrids first