As someone who has owned their own business (now retired) I like to encourage people to follow their dreams, be their own boss, and try to make a living doing what they enjoy. However, I'm not one to blow smoke up peoples backsides, and I believe that ones business model needs to be realistic. It looks to me that you are sincerely interested in learning what it takes to create a successful business, and that is encouraging.
However, I think you may have great difficulty in just getting a meeting with the owner of a restaurant. They get solicited all the time by a wide range of people. Just think how people tend to react when someone shows up on their doorstep, or calls them on the phone with unsolicited business opportunities? The typical reaction is- "No thanks, not interested", even if it's something they might have a use for. And those are the polite ones. Restaurant owners are no different. And just like the rest of the public, some restaurant owners are nice, and some are downright a-holes.
In my younger years I worked in several restaurants, washing dishes, busing tables, waiting tables, and what I remember is the owners either not being their, or being very busy. But one thing they all had in common was- they were VERY tight with money. At best most restaurants operate on a razor thin profit margin, with many just barely surviving, and that was before Covid. Trying to convince them to pay for knife sharpening could prove difficult.
Just walking in cold off the street, with no references, I'm afraid I'd be very surprised if you ever got the chance to demonstrate your sharpening skills. And to the average restaurant owner, particularly one who isn't a knife enthusiast, the idea of some total stranger offering to take out a knife in their restaurant, dull it, then sharpen it, might seem a bit odd and off-putting.
Over the years on this forum I've seen many members express an interest in making a living sharpening knives. But I've never seen one come back and say that they succeeded at it. That doesn't mean it's impossible, but again, you should expect it to be very difficult. Maybe focus on knives belonging to friends, family, and perhaps neighbors.
I wish you the best of luck.
David Martin uses a combination of Crystalon, DMT and India stones, primary customers are professional meat cutters I believe, but am not comfortable speaking for him, nor do I know exactly what his cash-work setup consists of.
