Which e-commerce app do you recommend?

I run a large Christmas Fair for my school. We have to list and sell over 100 stalls in various categories such as Christmas Decorations and Jewellery. Each stall can optionally have Power and Parking. Although I have 100 stalls I only have 50 power supplies and 60 parking places.

I have written my own application using WordPress and Formidable Forms but maintenance is hard work and I want to move to using a standard e-commerce package.

I need

* Configurable items with good stock control. If the first 50 people to purchase stalls order power supplies they will have to go out-of-stock.

* The ability to do PayPal refunds within the system. This is a killer at the moment. 5% of people cancel or amend their orders and everything has to be updated manually.

Ideally the system will be well documented/supported so that when odd requirements come up I can hack an implementation.

* Which package would you recommend and why?

Thank you for your help