If you have to start from scratch, you should have anticipated the issues and written the correct code. I have purchased one item in the last year and it was a hassle.
The market place attracted me to the site, which in turn attracted me to the forums, which had me look at the adds of your sponsors.. Now, I don't look at the adds or read the forums because I come on the site once per month to look at what's new and leave. A site is a total experience not just buying items. When one of the legs are gone, the rest suffers.
We all understand change and the need for it. I understand that your code was obsolete and believe you. What I don't understand is trying to make your customers get force fed these new changes. If I did that with my customers I wouldn't have any.
Most all of the members liked the old market place. If that is so, copy the old marketplace and write it in new code. If it is too expensive, then you should consider selling the site to a better financially heeled crowd.
The old tenant of business is you don't screw your old customers in the hope of getting new ones. Few business that try to rebrand to a different demographic succeed. I wish you all the best.
Respectfully
ChemE,