Random shit that's hard to buy. Think like rune daggers, adamant battle axe, Mith chainmail, adamant short sword, for basic random examples. Shit that people get as drops and just want to get rid of.. Unvalued items that hover slightly under alch value. They also have rough buy limits like 80-200. Once they get too far people will start to take advantage and ruin the margin.
Unless you resource something on your own, either the nats, or alch item, it's difficult to profit alching unless you're content with 30 alchs and hour with maybe 5k-10kgp profit.