I think 6 weeks is a totally unreasonable period between the purchase and complaint, but I also think I'd be very clear in saying a bike like that might need some work. I didn't read your ad, and maybe you did make that clear, but I don't think I'd assume a buyer would understand the bike needed work beyond minor maintenance. The tubes are ridiculous...if the tires are degraded I'd assume that new tubes are required as well.
The shipping issue has nothing to do with your problem, since you agreed to his parameters. Your time to address that was immediately after the sale when he mentioned the issue. Overall I think both parties share responsibility (depending on your ad) and the 30 offer was reasonable. Not everyone is reasonable. If it were me, I'd play hardball after a reasonable offer. I'd say something to the effect of, I acknowledge I should have been more explicit desacribing this, but I think you share responsibility based on the length of time between purchase and complaint, and your assumption it was ready to ride. I'd say look, I'll refund the 30...but I'd rather have it back then refund 60. I'd push the issue and tell him to ship it back for a refund and let him know should he go down that road, you'd open an ebay dispute and fight it out based on the 6 week complaint time.