First of, it is important to note that this is based on the fact that so far, every towny gave me the same trade offer whenever they came back, if they ever update their offers and add randomness to it, this guide will be outdated for the time being

Important to note:
Diamond, Gold and Ribbon rewards seem to scale off of farm level, so instead of number i will mention the relative amount you will get.
For gold, the "large amount" is twice the small amount.
For ribbons and diamonds, it is about 50% more for the large amount
As of Update #11, the numbers for the products now also scale with the farm level, for example Audrey's Salad trade now requires 290 Salads instead of 20 at farm level 136, while the diamond rewards is approximately the same.

Important
Update #11 changed the requests a bit, so i will have to check them all again, but so far it seems that the numbers just got bigger, both on the wants and offers
edit: went back in and checked a handful, general products of trades appear to have stayed the same, however the want-numbers are now big enough that i often find myself not having enough for them, while the rewards mostly stayed the same (or were even nerfed for some, like Lois' Ribbon trade now only gives a small amount of ribbons instead of a large amount)

In order to always be able to trade in the ribbon trades, you will also need to keep up with the Spice and Jam trades from Alfred and Rafaella, since these are currently the only available source of this ressource.
Additionally, you should always make sure to have some grain/wheat, some pears, and at least one of every flower petal in stock.