Like new is exactly as is literally said;
- the same as a new one
- or the same as it was when it was new
Sa for instance when its a plane it is like new when;
- one buys a plane, take it out of the box, carefully try it, does not like it, places in back in the box and puts it on ebay
- one buys a plane, uses it often and cares for it very well, replaces it after years and years of use for a different plane and put the old one on ebay.
- one acquires a plane, replaces all worn bits, redo any missing japaning, etc and puts it on ebay
For the first very obvious. The plane is actually still new, like when you go to the shop to see if you like the feel of a LN over a LV and you decide to go for the LN. The next person who buys the LV from the store gets a plane that's also like new.
The second is a bit harder. Is like new as like a new one from the stores which actually is a newer type after all those years because the casting dies has been replaced. Sure enough there is no difference to be seen as to like it was when first taken out from the box.
The third is very debatable. The materials used to replace bad parts are those of the same quality and is the machining of crafting on them of the same quality?