I'm thinking about buying a 70' self supporting tower that I can see had a little rust here and there. Besides minor rust is there anything I should ask the seller before I buy?
You should first ask the manufacturer. They should be able to provide the all-important safety, erection, and guying information.
Study the manual and relevant info first, which probably is available on the manufacturer's website.
I'd be more inclined to ask a local (metal) engineering workshop. I have one near me (Surrey UK) that do a large amount of work for a nearby amusement park. They would be well used to assessing the state of the steel-work underneath the galvanising (I guess it is galvanised). If you pay for their advise you should then have some come-back if things go wrong. I would also take into account where I was going to site it, in particular where and on what it would fall should the worst happen. Also - it is described as self-supporting - does this classification apply to all geographical areas? I am thinking here of wind-loading? A point relevenat for when you put aerials up there. Oh, and don't forget the base/foundation; the size of which is dependent on the last factor together with the type of soil in your area.