Also noticed the fur rug isn’t sellable here, but pretty sure I’ve sold them in the past, so are some items only sellable to certain traders?
You should be able to sell any of your items no matter in which shop. I think the shopkeeper level only affects items that you can buy, not the ones that you can sell.
However there are items that can’t be bought and / or sold. Looking at the firepit, that seems to be the case:
The fur rug should be sellable though. Did you craft it with the shop open, perhaps?
I’ve done a quick test and it does appear in the shop, not sure why it doesn’t in your screenshot 
Strange, kindof forgot about this, but that rug still isn’t sellable, and in fact seems to just be a ghost in my inventory. I tried placing it and running the game full speed for 3 days and noone ever placed it, also can’t find it in any of my chests. Shows up in the p inventory and the i inventory though.
Is there any reason why the fire pits cannot be sold? As the max level craft for the mason, it doesn’t make sense to me why this is so.
Because they are… firepits?
Yes, and?
I get quite a few from goblin camps and crypts and such anyway. Having to destroy them with an “out-of-game” command doesn’t make sense. We should be able to sell them off if we don’t want to use the extras.
You can also destroy them with an in-game tool, called Clear tool, on the harvest menu.
No need to use debug-tools…
You might want to post on the Suggestions category about selling firepits. I’m not sure about the criteria we use for deciding which items should be sellable and which should not, but we are open to suggestions.
Even having them sell for only one gold would be better in my opinion.