the problem is the barrel: noone wants the older plain 30" fixed choke 2 3/4" barrels. i would not pay more than $50 for one, and even then i would be inclined to just put that money towards a 3" chambered 28" vent-rib Express barrel with Remchokes (or whatever other type of barrel i was looking for). if you parted that Wingmaster out, you would sell it very quickly - but you would get around $200 for the receiver and $70 tops for the barrel.
i recently sold a good condition 2 3/4" Wingmaster with an 18.5" Express barrel (3" chamber) and 870P style wood for $330 -- but that barrel alone is worth $150.
IMHO the older Wingmasters represent the best value for 870 builds right now. the machining inside and out - and on all the internals - is light years beyond the Expresses of today. no dealing with the detent in the magazine to add mag extensions. all metal triggergroup. chromed bolt. no rust-magnet crappy finish.
$50-60 will usually cover the parts and labor to upgrade the ejector/ejector spring to a 3". you may luck out and have the smith able to re-use your old rivets. if not, and you are planning on refinishing the gun (parkerize, gunkote) then theyll refinish them smooth along with the receiver when you get the whole gun done. otherwise, honestly i would just leave them: you get two small rivet heads showing on the side of your gun - no big deal. dab some cold blue on them, and they get covered with your sidesaddle anyway. or just leave the gun as 2 3/4" -- theres no need for 3" shells in an HD/WD type shotgun.
i would not buy another recent production Express - i am very disappointed in the quality and finish. right now the only Express i have is a 20ga - its an eyesore with deep machining marks all over it despite the bead-blast/matte bluing, and i constantly have to oil it because of the rust-magnet finish.