Hi Matin!
Once a game is distributed as retail games to physical stores (internet or brick and mortar stores) , like AGEOD's are, its usual to find them at very different prices: offers, discounts, bargain bins...
If you want to support the developers its better to buy directly form their shop. They will generally 100% of your money.
If you want the best prices, you can find them on amazon, Play.com or other big stores, they are 100% legal. But AGEOD will get very few (if at all) money from it.
By the way, probably you already know but Amazon market place items are not sold by amazon but by other shops through amazon.
They are usually legit, but there can be exceptions.
The same on Ebay. Many seller are legal, but one can also fin pirated copies
Be careful about what you buy or where.
Cheers!