Every pea is a seed. Absolutely. But every pea should not be used to plant peas. Seeds you know nothing about, seeds you've gotten from your own plants yet you do not know both parents should not be used. I still gather them and hold onto them for emergencies?
Always purchase the best seed you are able to afford. Look at disease resistance, make sure the variety is able to thrive in your zone, know the pH that plant needs in the soil, how to water, that the drainage is exemplary, what fertilizer is the best and how never to overdo, how to build the soil you have, unless you are using pots then it should always be bagged, sterilized plain old cheapo potting soil...never soil from the garden.
Your seeds should always state NON GMO on the label. Gathering your own seed without the facilities to know both parents for sure, is too iffy.
There are lots of plants either 'feminized' or unable to make seed altogether to protect the patent. Peas aren't that great for eating as sprouts if that is what you mean by microgreens. I would certainly collect them and vacuum pack and put in the fridge or freezer. But I would focus on getting great seeds first...don't use anything you find in the grocery store
Peas are one of the easiest to grow, they love cold. Always start with non gmo seeds. Cost a bit more but you do not want genetically modified pea plants...trust me.