Launder your items first, that way you can't lose them, then sell the ones you have left over. It isn't hard to avoid selling stuff you want to keep. Even if you launder something by mistake, you can still sell it.

I do the same.

When I get to the fence I switch to Launder and go through each category of items checking for things I want to keep before I sell anything. Which works well since all the treasure items go into Miscellaneous, meaning I get to them last.

Oh, except I skip Materials, since that can easily use up all of my laundering slots for the day, but there's relatively few I really want. So I save them till last and sort through from the ones I have fewest of to the ones I have most.

(This did once cause the opposite problem - I laundered about 3 or 4 items I actually wanted to sell because I forgot to switch tabs, but that's better than selling something I wanted to keep, it just meant I lost a couple of hundred gold.)

I sold a couple things once thinking I was laundering. After that one time I always sell/launder by sub-category so there's no mistake.

It sucks, especially since there's no buyback, but it can be avoided.

Don't forget the Fence has a Furniture tab now. But yes, launder first, fence second.