An electrician was at my home and did a check after adding a new large plug for an electric range. He showed me the breaker panel in the garage where there are some burn marks and said it needed to be replaced. Conservative estimate is $3500. Do burn marks on the flat metal that the breakers connect to indicate a real problem and if so does 3500 seem reasonable replacement cost

Yes there may be a problem but without more expert opinions you will never know if his price is reasonable, or entirely necessary. Get a few more estimates at least.

I agree, more estimates are needed. Rarely does an entire panel need replacement