The I2P network is similar to Tor but DDOS is not possible on it.
Markets could provide an alternate way to access the market through I2P alongside the Tor hidden service. This way the market can be reached through Tor and I2P, so even if .onion is down users can still reach the market on I2P.
Using I2P might be a little bit harder than firing up the Tor Browser Bundle, but in times like these it would worth those extra steps to access a market even when it's under DDOS on Tor.
We had a good i2p marketplace, multisig, smart admins, the whole package. Nobody wanted to use it though because 'durr i2p is hard' so they closed up shop.
Also DoS attacks are still possible, there are some good fixes for certain known attacks but it's a young project and lots of stuff would pop up if everyone started using it.
That being said diversifying the marketplaces and more markets in general is a good idea.