Playing Skyrim Legendary edition (pc) Normally when when you first arrive at Riverwood your approached by a cloaked figure who is relentlessly compelled to talk to you to begin the dragonborn dlc. I have successfuly managef to dodge this npc, and not talk with any of the guards. Sometime after reaching level 20, guards no longer tried to mention the Dawngard questline.

Now, to the problem: I've completed the "Laid to rest" quest in Morthal (I'm a thane there) and talked with both the yarl and his stewart and neither one will offer to sell me a house / land. Earlier I bought Breezehome without any problems. Is this a bug or am I over-looking some other requirement?

Aslfur should be giving the dialog option to purchase land if you're thane already. Unless you've started another "Build Your Own House" Hearthfire DLC quest in another city (Falkreath or Dawnstar) and haven't gotten through the 4 easy objectives yet, I'm not sure why the steward won't offer the land sale to you. If you're sure you don't have another Hearthfire house quest running, then you can use the console window to force it to start.

While in game, hit the ~ key to open the console window and type in this case-sensitive command: setstage BYOHHouseHjaalmarch 30

Then hit the enter key, and then ~ again to close the console window. That should jump your house quest to stage 30, where the objective is to talk to the Steward about the land for sale. If the objective doesn't show up in your quest log, either under a Build Your Own House quest or in the Misc quest section, then the quest may not be running for some unknown reason. The console command startquest BYOHHouseHjaalmarch should fix that.

EDIT: Be aware that receiving and reading the letter from the Yarl of Falkreath after you've reached level 9 may possibly count as having another Hearthfire quest running. That letter is about getting the Build-A-House in Falkreath and reading it does advance the quest stage for that hold. If that's the case it may be possible to resolve this merely by obtaining the Falkreath land first, before asking Aslfur about the land in Morthal.