adding a “sold property” icon

I'm guessing that you probably mean "a map" rather than "the map" - there are lots of maps incorporating OSM data. If you want to create a map with markers on it indicating properties that have been sold I start with the switch2osm "using tiles" pages. One of the examples there is a property website...

https://help.openstreetmap.org/questions/25/how-do-i-add-a-marker-to-a-map

I would not tag sold houses here as apart from a few new unsold houses the rest ARE sold so just tag the "for sale" ones. If you want to show the houses you have sold in an area you could use OSM data as a basis for your own map or web site with your properties highlighted. The documentation tab may give some info on how to do that.