Lets give this a go....
1. Why didn't anyone eat the cake in the first place?
The reason no one ate the cake is because its a collectors item, quite rare and eating it would just be a huge monetary loss.
2. Did they buy the cake with the intention of selling it later?
No, they fully intended on keeping the cake forever like those folks that have the 1950's Twinkies.
3. Where did they keep that thing all this time?
It was tucked away in a container with ponies down at their none climate storage facility.
4. How in Equestria did they think someone would buy this?
Easy, people buy a box of 1970's baby diapers for some reason I can't think of, people buy 1970's candy, people buy a piece of toast that looks like Mary and last but not least....people buy other people's dead beat husbands. So why not a cake?
In the end, I think the only question I really have for this seller is, why?
There is a point in pony collecting where all that glorious sea of symbols, sweet smells, colorful manes and cute little eyes send people over the edge. There is a gene that some people have where after collecting MLP for long enough they begin to lose their thinking process. Everything is pony to them. Nothing seems impossible. They have magical powers like unicorns. So, in their mind's world anything and everything pony is perfectly normal. They go down to the store to buy a cake because hey its got a cute little pony on it. They hoard it for years and hide it from their kids because its just so special. Its so valuable they don't even keep it in their home. Then one day their eyes see stars and cash falling from the sky. They decide since they have to now pay for college tuition; the cake must go. They sell it.