Matthew, there are two Icom 700 models active in the boating fleet right now. The ''Pro'' replaced an earlier model perhaps 5 years ago. I would suggest you ask the seller to confirm they are selling you the ''Pro'' which I''m guessing they are not.
The earlier 700 had a good rep and held up well in the marine environment but that model is not suitable for pactor (email), is probably fairly corroded internally by now if it''s been on a boat, and I would suggest you buy it subject to inspection and repair. The idea I''d consider would be to send it to AES (as one example - Icom dealers, certified Icom techs, etc.) for a complete bench test. You pay for the test (like paying for a survey...) but the seller pays for broken parts or declines the have it fixed at his/her expense, in which case you walk. I think you''d want to have the tests done even if you bought it outright.