Unless you live in a very humid area, the battery corrosion is not an issue. I've cleaned numerous Marks in my collection. Original Mark III BD, Original Mech Test Tony that have had battery leakage in the chest and arms. A q-tip and hydrogen peroxide is all that is needed to clean the contacts and it is good as new.
Considering the Mark III Silly Thing was never sealed in production it is not fair to say all MISB are not new and never displayed. YMMV. i think you are living in a dreamland saying $700 is a fair price for the MKIII Gunmetal given the low production and that it is probably the most expensive HT selling on ebay right now ( at least Top 3).
If I needed those two pieces in my collection, I think a fair price for me is $1,500 for the MK III Silly thing and $650 for the MK VI if it is sealed in Shipper. But the real answer is whatever a person is willing to pay. We have already seen a few ST Mark III's sell for over $2500. Don't be so quick to judge on the want or rarity for a completest, logic falls off the truck sometimes.
