You are correct in that without pics it's hard to determine value. 1st, you need to verify that it's a "true" 28 ga (approx. 5 k made--probably more than that remade from 20 & 16 ga guns). Serial # is a good place to start--however you should perform even more research to verify it's manufacture (ie/stamp on the back of the receiver etc--even that can be faked).
They are great guns to shoot--and even a "made up" is still a nice gun.
As always,an original is worth more, even w/ cutts and an aftermarket rib.
If you can buy it "right"--go for it and enjoy--if you desire "real"--research it and be willing to pay (do you want a shooter or collector). Of course, wood and condition always play into the value.
Here we go,----want a great alternative--for a lot less $$--buy a browning model 12-28 ga and have choke tubes installed--probably better steel and absolutely manufactured to tighter tolerances. A fun gun to shoot ! (ps, -I don't have one for sale).
regards
Bob