Custom texture packs display unavailable deals as available. It does not seem to be texture pack specific issue. All texture packs I tried have the same issue.
I solved the issue temporary by extracting the texture pack .zip into a folder and deleted gui\trading.png.
I did some more research on this. It is not a Minecraft issue, but poor design on most of the texture packs. trading.png has "available deal dialog" texture on left and "unavailable deal arrow" on right side of the texture file. The arrows are very similar in most texture packs. Sorry about the trouble..