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 (she has two), players must be certain to keep an eye out for the level 20 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 15 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 Farmers around the area, she will yell, "You're the one who's been pickpocketing the farmers! Guards, guards!" On either occasion, a level 20 market guard will attack you.