I winterize weekly by draining FW tank, HW tank, open kitchen, tub & shower faucets then open 3 line drains. When everything is done draining I close it all up then add a cup of RV antifreeze to each of 3 drain traps and run some antifreeze through the pump. Oh yeah, if I think of it (most times I don't) I'll open the toilet flush valve and let water drain back into the supply line. This takes about 15 minutes. The next Saturday I fill it up again, take a trip and drain again Sunday night. I don't blow the lines, I don't fill the lines with antifreeze.
A few weeks ago we had a very cold Friday night . . . teens I think. When I went to water up Saturday morning the outside supply intake was frozen. Eventually a trickle made it to the (now open) nearbly line drain and got 2 ft of intake line flowing. So crank up the propane + electric heat and open up all cupboards, drawers, bed, etc and wait. After a few hours finally got some flow to the nearby bathroom faucet, then tub, then another drain valve and finally to kitchen. At some point finally got water to the HW heater but noticed the tup drain was stopped up (ice). Found an access plate and hit the trap with a hair dryer.
Eventually I was on my way . . . to sunny Florida. woohoo!!!
(but I was back home Sunday night . . . ugh)
I know the supply lines have some expansion capability so that must keep them from splitting from the residual water (temps have been in single digits) but I'm very fortunate not to have split the trap.
That being said, perhaps the seller didn't do a full winterization and knows there might be some residual water in the lines. Dunno, just a guess.
Keep in mind I do this weekly, arriving home at 10pm or midnite (or later!). It's not practical to do a full winterization every week and I don't have access to compressed air. I do have an air bottle but blowing the lines uses most of it up . . . . .
But I wouldn't buy it without a full test either. Pretty good bet nobody else will be either and Spring is just around the corner.
Good luck,
- bob