I haven't been playing long at all and my inventory is already full. I'd like to sell a few things but cannot find a store or merchant anywhere yet. So far I have only been to the towns of Concord and Sanctuary, is there anywhere around those where I can buy and sell some things? Thanks.

Once you get in a populated city (I don't want to spoil) you'll find plenty of merchants. There are also multiple wandering caravans walking from settlements to settlements. Don't worry you'll figure it out pretty quickly, you haven't explored enough for the world to make sense yet

Ok thanks but can you please tell me where one is around the locations I've found? I desperately need to sell some stuff and also need to buy some supplies. Thanks.

But there isn't a town near by sanctuary with a merchant? I really need one.

There's a fireplace on the east road out of Concord where traders stop. they also stop in Tempines Bluff. (East of Sanctuary, Preston gives quest for it)

Drumlin Diner, it's south east of Concord, little trader post set up there.

In the meantime you can stockpile your surplus gear in Sanctuary - build a box and stuff it all in there (or store all Trash in the Workbench anyway). You can of course break down ordinary weapons and armour for crafting supplies but you may want to salvage weapon mods. To do that you will need to craft a "Standard" mod for each part you want to take off (probably will use a lot of adhesive) and the mod will pop into your inventory. Store them in the Workbench.