be able to help if he has a model number to work from; repair kits are often available for such work and contain instructions. it was just such a salesman that, given the exterior parts of my faucet, was able to point me to the parts i needed for a repair job.
once all the information is gathered, it is time to begin work. don't be afraid, you can do this but only if you can convince yourself to start. as your work will take a while it is wise to provide for plenty of time if possible; don't shut down your entire water system not knowing when it will be 