you could take a look at openbazaar, which (as far as i know) is the leading decentralized market software currently being developed. it uses cryptocurrency for payments and ipfs as a filesystem.

you surely could use the blockchain as a database, because you can attach small amounts of arbitrary data to blocks[1], but it would be very inefficient (transaction fees!). something like ipfs is probably a better solution.

i'm not a software developer, i just think this stuff is really cool and follow projects. someone with more technical chops could definitely give you some better pointers! i do have this bookmarked, though: https://dappsforbeginners.wordpress.com/

[1] https://poex.io/about