I honestly believe that at this point in game running a shop is more time consuming and less rewarding then it once was...
Fight alittle to acquire an arty or a PS and you got about 10 mil, you spend the day making kegs, apples etc and you make a couple of 100K
Gone are the days of selling a full Valorite platemail suit for 25K and thinking you were rich.
I don't recall them asking what to sell to make the most money.
There are shop owners who sell just for the fun of selling things. I have a pretty time consuming shop, I spend 90% of my UO game time doing things directly related to it. So my advice to the OP is figure out how much time you want to devote then go on from there. There is nothing worse than a shop that never stocks. If you have lots of time you can get into the potion/repair deed/scribe/engraving tool business, the returns are poor but with a regularly stocked shop you will get business and lots of returning customers.
Also consider how you spend your time now and what you enjoy doing. If you like growing plants you can sell the spares. If you like killing things you can harvest leather to sell, if you like LJ or mining...etc. If you fill lots of bods but don't use all the rewards make a reward sales vendor. If you dont' have time for that you can still get bods and just sell them as is. You can always make some money selling junk bods for people to refill and good bods for a bit more (tip: sell the whole books of junk bods in lots of 100/200 etc).
Weapons sell poorly though that might change a little when SA comes out. Armor sells ok as long as its pretty decent, jewelry can sell ok as long as its decent to really good, etc. Since people are going to need stuff to imbue you could make bags of 25 junk items to sell for 5k gold or whatever for people to unravel.
You can do furny or stoneware vendors, tho those can be very slow sellers, especially on a lower population shard. Though SA should boost furny sales with new and returning players needing to spruce up their houses.
If you *are* going for gold, currently the most money I make in a consistent fashion is off of my tailor/smith bod reward vendors. Runic hammer sell really well. I also get nice items from camping idocs, then I can resell those low and do well, but for most players those aren't really something they can do regularly or often.