How to create a form with ~15 fields for a buying used clothes site, single or multi page? I saw other debates & older forums, but im not still convinced. Im leaning towards one page, or maybe accordion.. Is there a latest AB testing study?
I would say Accordion.
Multipage may give issues when going back a step and losing form data, plus it's usually not possible for customers to 'look ahead' to what they have to fill in. It can feel too opaque, like you're expected to sign a contract but they're not letting you read it in its entirety.
But a single page can seem daunting, especially if you have many fields. The 'fold' still matters on websites.
An accordion would keep users focused on the portion that they currently have to fill in, yet gives them peace of mind because they have freedom to look at the whole process.
Also, try to keep the form chunks consistent if you decide to somewhat separate them. Imagine if http://www.getelastic.com/wp-content/uploads/treatment.jpg was split up, the first page would have 8 fields for shipment, and the second page only a single toggle. Instead they should have asked for communication info (first name, last name, email, phone, newsletter), followed by delivery info (address, what handler to use). More evenly sized chunks and better contextualization.
You say you've got 15 items, so 3x5 would be a nice grouping.