Does it ever seem like things are backwards. These are some things I noticed working on 3 rail trains over the years.
The cheaper the 9v battery, the less likely it is to leak. I have bought trains with top name brand 9V batteries left in them for 7+ years and they leaked. I just pulled out a super cheap no-name battery with a July 2012 experiation date and not a sign of leaking. Might make it through the next ice age without leaking. My VOM showed 9v still, but not enough current left to run the sounds at shutoff. Dead, but not leaking. Maybe there isn't as much to leak out in a cheap battery?
The bigger the item, the more oversized the box and documentation will be. I have a K-line bigboy from 2002 era. The original box is about 1.5-2 times bigger than it needs to be in my opinion. The documentation instead of being a 5.5" x 8.5" I normally see with K-line, it is a full 8.5" x 11" booklet. Maybe todays shipping cost will reverse this trend?
When you try to buy trash, you sometimes end up with gold. I needed a truck for a 1950's madison car. Found a parts car for sale cheap, cracked body, windows knocked in. A little fuzzy on the pictures. The trucks looked good enough in the pictures. When I got it I found the frame and trucks in pristine, unrun condition. Light bulbs in new condition, only a little dust on them. Not a run mark at all on the wheels or pickups. If there wasn't a chunk taken out of the shell it would have been good, perfect screw holes still. The wires had gone hard else I could have used the frame and wheels as they were. Also the window frames didn't have a spec of rust on them. I replaced the window silhouettes since someone poked them out. A spare shell and I have fundamentally a new Madison car.
The more you try to save money the more it costs. After a couple of, "I can fix it" projects, I no longer buy them if they are not complete. Complete and not running, I can fix. Finding missing details, trim or parts often ends up costing more than buying one in good condition.
Don't buy one, hoping to find the other. Example, I have a TMCC powered UP F3 B unit, it was a deal. Finding a dummy A unit or even a matching A unit shell is like finding hens teath.