Common sense advice from most people would be to just buy as there are quite a number of choices out there. Well, not everyone thought the same. These guys went and built their own ERP from scratch. I still believe there is much value for developers to go ahead and building your own (especially for a small business) as it is an invaluable enterprise development exercise, just as Linux geeks see it as a rite of passage to build their own distribution from scratch.
Rolling Your Own ERP by Dennis Howlett