The shop in Ingestre St opened two and a half years ago and Ms Sibbald says they have built up a loyal customer base.
"Some people have a perception that our sole purpose is to provide cheap goods for people when our primary aim is to support our charity.
"We are raising money to fund our ambulances, paramedics and community projects.
"Then we have to meet our overheads - rent, utilities and other costs.
"We do research to see what goods are worth in order to price fairly.
"People donate things to St John and they know if something has value, they hope we will get the best price for it to support the charity.
"You get to know what people are willing to pay and if things don't move, we reduce the price."
Regular customer Fran O'Leary said she loves visiting the shop and finds a lot of good bargains.
"I'm not usually looking for anything in particular but I usually find things I really like and I also enjoy coming in to talk to Fiona and the other workers here."
Hospice Wanganui has three charity shops in Whanganui and the new boutique shop in Victoria Ave sells the "high end" goods with bigger price tags.
"Hospice cares for people who are dying and people donate things of value because they want to support the charity," said chief executive Karen Anderson.
Ms Anderson said label clothing and accessories are a good second hand market.
"People pass on things that they have treasured in the hope that they will fetch a good price for Hospice," she said.
There are ten Whanganui second hand shops listed on the online Op Shop Directory.
Most of them are run by charities and there is also the Koha Shed in Whanganui East providing free clothing and goods to those in need.
The Wanganui Chronicle investigated some average prices of winter clothing accross three stores.
+ Scarf prices ranged from $1 to $5 and there were a range of acrylics and hand knits.
+ Cardigans and jersey prices ranged from $2 to $20 for a near new hand knitted jersey.
+ Hat prices ranged from $1 to $5 for warm hats.
To find opportunity shops in the region visit the Op Shop Directory.