What would new tires, tubes, brake pads, seat, and tuneup work cost me?
Hi Manny - I think the waiting advice is not bad - a better deal usually comes along, usually right after you just spent your money!
But for reference, some inexpensive tires on sale (Vittorias) say $30-35/pair, tubes $8/pr, brake pads $12, seat $20 (although you could probably get one for free if you have some biking friends), so maybe $70 total. The best would be if you can do your own work. Tuneup would be to install the parts you bought, lubricate the cables, adjust the hubs, true the wheels, adjust the shifters, maybe wrap some modern bar tape (that's another $6

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$200 was probably the top price for that bike as is, so it's good he turned you down. When a CL seller has rejected my offer, I usually tell them that at least they now have an honest offer, to keep my phone number and call me if they want to reconsider.
For reference, I just bought a nice Motebecane CX Pro cyclocross bike, SRAM Rival equipped, for $500. The bike needs nothing. The guy originally was asking $800 (sold new for $1200). When it didn't sell, he re-listed it for $600. I called and asked if he would consider $500 (a low offer, not a crazy low offer). He said yes. We met 30 minutes later. Looked at the bike, rode it, peeled off 5 bills and put it on my car rack.