There are three traveling salespersons you can meet who will offer to sell you rare items. They will greet you when you exit your farm house between 14:00 and 18:00 on non-festival, non-stormy days starting in Fall of year 2. You will meet one of the salespersons when you exit your house, then some time will pass, then you can bump into the next salesperson, wait some time, and so on. The three salespersons are Dmitri, Rainer, and Susanne.
At a minimum, there will be 20 days between sales greetings. This means once you've been visited by a solicitor, at least 20 days must go by before you have a chance of being greeted when you exit your farm house. If for some reason you don't trigger a salesperson within 60 days of your last visit, you'll automatically get one when you exit your house during the time they're available.
When you are greeted by a salesperson, you'll see an introduction, the item name and quantity that is being offered, and then the total price. Some of the items they have can only be purchased one time, such as blueprints or cooking recipes, but others are part of their stock inventory and you could see it offered more than once. The quantity of the item being offered will be between 1 and 20.
You can decline to purchase a salespersons's item without penalty. The next time you receive that salesperson you may be offered a new item or, if you're unlucky, you might end up with the same one again. The item offered is completely random from the salesperson's inventory. You have no control over what is being offered during that visit. Saving your game, triggering the event to see what you get, and then reloading your save game will not result in a new item for sale, but it can change the quantity offered. For example, if you save your game before exiting your house, and Dmitri shows up to offer you 13 Pink Diamonds for a total of 68,250,000 G (yikes!), you can then reload your save game to trigger the event again; this time Dmitri might only offer 3 Pink Diamonds, but he will always offer Pink Diamonds until the next time he visits.
Playing the game in Seedling mode will NOT decrease the traveling salesperson's prices by 30%.