In this particular case, I'm not so sure that the seller IS BREAKING UP an engine/tender set. The locomotive is #2843, and the tender is #329(Union Pacific).
I suspect that these were not originally a paired set. They MAY OPERATE together alright, but I don't think that they were originally shipped together. There is a pic in the auction that shows the owners manual, and it clearly shows the model available in SP as well as UP. My Lionel UP versions of this model are #ed in the 32X series, and the SP versions are #ed in the 26xx series, so in this case, I think that the seller is selling a UP TENDER, and an SP LOCOMOTIVE in separate auctions, NOTHING WRONG with that, IMHO.
Something else that I suspect happened here is that the SP TENDER, and the UP LOCOMOTIVE, may have been accidentally sold previously as a set, either by this seller or a previous owner, and now, there is NOT a matching set to be sold as a set, VERY possibly in an estate sale situation by someone not train savy.
This seller may well be AWARE that these are not a properly matched set, and is selling them separately for that very reason, just think how many "Look at that IDIOT, selling a UP tender with an SP LOCOMOTIVE" comments he would get selling them as a set. I think that the seller is just trying to sell off some mismatched equipment in the best way possible.
Just MY thoughts anyway, YMMV
Doug