Both has been proposed to be be merged with each other … and NON result yet … 
shop=lighting or shop=lamps
Shop=lightning is a shop where Thor is peddling his wares? 
… and this is a problem why exactly?
Taginfo has one used more than the other - https://taginfo.openstreetmap.org/search?q=shop%3Dlighting https://taginfo.openstreetmap.org/search?q=shop%3Dlamps . The terms aren’t synonyms, though there is some overlap.
sorry, my mistake, edited it … 
shop=lighting seems to be more used, and also, a lamp shop IS a lighting shop … isn’t it … so why not merge it then ? 
solved