This is a Configurable mod which adds inventory to any shopkeeper. This is done via a json file called config.json, which is included in the mod folder.

Here's an example of a very simple config.json file:

"shopkeepers": {
"Marnie": [
[ 174, 300 ],
[ 182, 300 ]
]
}

In this example, we're adding two items to Marnie's shop. 174 (a large white egg) and 182 (a large brown egg), both of which will be sold for 300.
You can also add to multiple shopkeepers by adding another section, like so:

"shopkeepers": {
"Marnie": [
[ 174, 300]
],
"Pierre": [
[ 182, 300]
]
}

Now Marnie will sell the large white eggs, and Pierre the large brown ones.

Update (as of 0.1.2) :

You can also specify the year and season you'd like the items to be available, such as:

"shopkeepers": { 
"Marnie": [
[ 174, 300] 
],
"Marnie_Year2": [
[ 444, 2000]
],
"Marnie_Year3_Season_summer_winter": [
[ 446, 2000]
]
}

This config file will add a large egg to Marnie's shop from the beginning of the game, all year long
It will add a duck feather starting in year 2 (and every year after that)
And then will add a rabbit's foot in year 3 (and every year after), but only in summers and winters.

These are all additions to the shop, so in the winter of year 3, all 3 items will be available. 

All object id data can be found here: http://canimod.com/guides/object-data

The source code for this mod can be found here:https://github.com/IrregularPorygon/SMAPIGenericShopMod

For seasons, it's important to keep them lower case, adding _winter will work, _Winter or _WInTeR won't. 

Initially, this mod only adds farm animal products to Marnie's shop. Those can be altered and replaced at any time by editing the config.json file as shown above.