The resellers we list are big, and generally have good feedback. The contract is still with the main network, so you get the same service, just via a cheaper source.
In the unlikely event a reseller went bust, don't worry - your contract will continue as normal as it's with the network (so there would be no change to your price or allowance of minutes, texts and data). The reseller is responsible for consumer rights on the handset though, so if there's a fault you'd need to rely on the manufacturer's warranty, or consumer rights via the payment card used.