You can use the 1901 census to locate William and Elizabeth in Gorton, probably on their own as they will have married around 1896-1901, check the marriages on the free BMD index on ancestry or try here:
http://www.lan-opc.org.uk/Search/indexp.html
That's where I found the baptism in 1901 and no other entry in the index came up for william arthur sellors, possibly they married in a church/chapel not yet in the index, or possibly william is indexed without the arthur included.
You can check the free BMD for William jnr's registration which will give you the registration district you need to use to look for any later sellors births registered in that same district after 1901, or, just go straight to the 1911 census and try to spot the family on there, assuming William Arthur snr and jnr were alive in 1911, possibly Elizabeth had died by then as you say William snr remarried.