The bug where it saying your bag is full is that the item in the Junk Man is sold out. What happened there is someone bought out the last of that item, in which case the listing should disappear, but show's with the message that you don't have enough inventory space. It should say something like "This item is sold out".
What I've noticed is when something is bought out the listing doesn't actually disappear, instead just causes you to mistakenly buy the next available item that's in stock.
Seem's to be that the problem is the NPC is having troubles clearing it's item listings when players buy out a stock completely.