I've learned a long time ago that if you can't see it, touch it, hear it and smell it, it's not worth buying. Treat this like a scam until you see the machine in person.
I really should re-read the ads. The seller mentions the machine likes to walk across the floor. When you go to see the machine, check to see if it is bolted down. If not, I'd wonder why.
The seller is bidding high because it's an antique. If I were in your shoes, here's what I would do:
If the cabinet was in pristine shape and you don't see any rust, I'd offer $150 for it and keep ramming the fact the timer is hosed to them down their throat. Who knows how badly the outer or inner tubs are rusted until you tear the machine apart.
If the cabinet was damaged, dented or rusty, I'd probably either offer $50 or walk away, because that machine will cost you a lot more to restore.
The dollar amount suggested in the ad is an arbitrary figure. As much as I hate to say it, it sounds like the seller is nearly ready to scrap the machine, so take advantage of that and use that to get a decent price, assuming it's not junk.