You've got it just a little bit wrong way round, the Webley stamp is the original production starting around 1915 and the Enfield stamp was the later production in that 1923-26 range. I have observed values in Canadaicon to range from $400-700 CDN, so if you are seeing gunbroker prices at the $500 USD point that might not be too far off what is threshold for your market.

Doing clean up, it is important not to disturb the finish, but rust removal will always benefit a firearm. Do a bit of research first but you will generally find that fine steel wool and a proper gun oil is the best route for all but the worst condition guns. Avoid using mechanical means like dremel tools, abrasives like emery cloth or sandpaper, and acids like Naval Jelly.