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.
Also, sort by price as most items you would want to keep are not 40, 100 or 250 Gold. Of course if you steal armor and weapons those are different prices and make this harder, but even then, they tend to group by value.
Enjoy the game, life is what you really want to be worried about.
PakKat "Everything was going well, until I died"
Gary Gravestink "I am glad you died, I needed the help"
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.)
PC EU player | She/her/hers | PAWS (Positively Against Wrip-off Stuff) - Say No to Crown Crates!
"Remember in this game we call life that no one said it's fair"
If on PC use the add-on "Dustman". Takes a little bit to set up the filters just right but once you have it set it will auto launder the items you want and auto fence the ones you want. Also works for regular merchants so you can auto sell the junk.