Sometimes when I talk to the blacksmith chick in Whiterun at just the right second between her lunch break and going back to work, all she'll have to sell me is an odd collection of food items: I presume this is her lunch.

It is, indeed, a placeholder, and not meant to spawn in his inventory. It doing so is, as best I can tell, a glitch in the processing of the vendor trunk script.

Do NOT try that, nor attempt to sell it, as doing either can hose up that game. Do not put it in your house, either, but find a respawning container and stick it in there so that the next cleanup cycle safely removes it. This thing is not meant to be in play at any time, and needs to be disposed of ASAP in order to avoid future issues.

I saw an item in someone's inventory called "no name defined" or something similar.

I have seen this, as well. Very strange.

You're going to need to substantiate that claim, my friend. From here it looks like hysterical conjecture.

Merchant inventories have been done like that since morrowind. if that chest were to disappear he wouldn't have anything to sell.

WOE BE UNTO HE WHO DELETETH THE CHEST WHICH SAITH THAT THOU SHOULDEST NOT DELETETH IT. FOR ON THE DAY TOU DOST DELETETH THE CHEST WHICH SHOULDEST NOT BE DELETED, THOU SHALT SURELY BRINGETH FORTH PESTILANCE, DEATH AND FIREANTS UPON ALL OF TAMRIEL. AND HE WHO DARETH TO DELETETH THE CHEST SHALT BE DELETED.

Or just have to start over from scratch, meh.

Mine sounded better

So I've kept the chest in my inventory and it's not affected my interactions with Belethor at all. I can still buy and sell from him normally and his inventory still refreshes and changes in the normal amount of time. And to whoever asked earlier on about the item ID or something... sorry, I'm on PS3 so have no way of finding that out!

It's not hysterical at all, it's based on experience modding inventories in Bethesda games. All merchants use leveled lists to determine what may spawn in their 'sell' menu, and items such as the one the OP is describing are often used as either script placeholders, templates for other items, or (rarely) both. Said 'base' items are not meant to actually be pulled up when a respawn is called for, as the game uses them internally. Thus the 'do not delete' tag, as deleting the item can/will cause problems, up to, and including, crashing the game when it cannot resolve a script check and hangs.

As for the 'get rid of it right away' comment; having the item at all means his game has glitched, and the best way to fix it is immediate disposal of the item he shouldn't have. The best way to do that is put it in a respawning container, since then the automatic cleanup routine will properly remove the item from play, which clears the glitched state since the item is no longer there to cause it.

I'll admit my presentation wasn't the best, and the OP has so far not experienced any issues from keeping it, however I felt it wise to warn the OP of the potential consequences of keeing the thing around when it's not even supposed to be there in the first place.

So in the very worst case scenario... he'll lose the ability to trade with Belethor? Hardly a game breaking consequence considering all the other bugs this game has.