I do this, don't see the specific item I want, then f*ck off to try again at 6 tomorrow.

If you're on PC, you can noclip (command: TCL) through the floor to the vendor chest, and just add 10k gold to it (command: player.additem 0000000f 10000).
The command adds it to your inventory, then you put it in his chest. If you then sell your items to him, and get the non cheaty amount of cash and can have a "clear" conscience.