I imagine the "sell-sign" issue is because of semi-petspective primacy of the Depot. The dwarf off-depot that is layered difectly over the grass is then over-layered with the "nearer" depot elements, including the buy-sign, that poke up into those tile-boundaries. The one in the depot is atop the depot, which has already been constructed of the standard floor, furniture (stools and tables) and the buy/sell-signs as increasingly viewer-wise layers before tabled goods and character figures (including that entabled dwarf, you mention, clearly being offered for sale...
) get their own overlaying.
The solution might be to 'reserve' the non-diagetic signage for even-toppermost over-layering. Or to rework "any character in the footprint of a workshop/etc" out of the more general landscape and slot them into the workshop's layer-construction scheme[1] as a localised under-to-over and rear-to-near stacking to produce the completely accurate 'occupied Depot' stack (front tables and signs show in front of such midway-standing characters) to
then place onto the ground (showing in front of the characters standing just beyond the rear). But there might be other considerations[2].
I mean, I'm just guessing at the current internally-coded approach, so my 'solutions' might be starting off from the wrong premise. I could just be putting the
cart before the horse wagon before the yak[4]!
[1] In this case, it looks like
only the buy/sell-signs poke over tile boundaries beyond (the five tables, and the two bargaining stools, stay within), but some workshop worktops/decorations might also extend. And some creature-sprites (esp. Giant <Foo>s) might themselves poke downwards and sideways in a manner that should go 'under' those tabletops..?
[2] Going with the possibility of Giant Foos being widthwise beyond the tile boundary, you'd want one just off the side of a Depot/similar to layer above the flooring element but sneak beneath the table-top? That'd require keeping layering as a deconstruction (which would also solve the sign) by putting ground-texture, specialist floor texture (first constructed, then 'built room', if constructed isn't already subsumed into earth/grass/etc ground), grounded furniture and creatures[3] with rear-to-near overlaying enforced for all overlaps and perhaps a firm lateral policy for the Giant things vs tables, then repeat for things
on the tables (goods, buy/sell-signs, workshop-worktop-tools, etc).
[3] Further separating potentially flying creatures? And the 'table-top' dwarf ought to be deferred at least until the sign's level (if in the same tile) to appear
on the table where casually allowed to walk across it, as apparently this one is.
[4] Which, with reference to recent "face left or right?" discussions in mind, I expected to be the main point-of-order in discussing this particular release of images!