She sells a range of seeds fit for lower level farmers, the list of which may be found here. There is an option to ask about high level seeds; she says Master Farmers have them but won't give them to you, but many of these she also has but not for sale - you can steal them from her stall.
When stealing from her seed stalls (2), players must be certain to keep an eye out for the level 42 market guards, as they will attack the player if they are caught. If Olivia sees a player stealing from her stall, she will stop them, shouting, "Hey! Get your hands off there!" and they will be unable to steal from it for a short period of time.
If a player has not been stealing seeds locally, they can sell seeds to Olivia as well as buy them.
If a player steals from her stall, they will be unable to buy from her store for fifteen minutes. Like most stall operators, she will yell, "You're the one who stole something from me! Guards, guards!" On another note, if a player steals from the Master Farmer around the area, she will yell, "You're the one who's been pickpocketing the farmers! Guards, guards!" On either occasion, a level 42 market guard will attack you.
A vial of stench (a) can be used with Olivia by right clicking her and choosing trade. This will not work if the player has tried to talk or left clicked her if they have pickpocketed the master farmer or Martin recently.