From a consumer law perspective, buyers have a right to a refund if the item has a major fault that was not disclosed prior to purchase (they can also choose a repair or replacement if they want to).
If the item has a minor fault, the rememdy is at the seller's discretion - they are obliged to provide a rememdy, but they are legally allowed to refuse to refund and instead offer a repair or replacement (unless their return policy actually stated that refunds can be provided, then they are legally obliged to honour that regardless).
What actually qualifies as a "major" fault varies depending on the item, usually it's something that significantly affects the long-term use of the item's function, and would have meant the buyer never would have purchased if they'd known about the fault up front. eg A loose or missing button on a shirt = minor fault, easily repaired with little to no effect on the shirt visually of functionally. A torn breast pocket however would be a major fault, because even if repaired, the item still wouldn't look right - so, while probably not a perfect example to compare a GPS unit to, I hope it helps clarify whether the fault with your unit was minor or major.
