The 30 items a year isn't a cap, that's the reporting threshold for eBay to report your sales to HMRC.
From 16th October eBay will keep payments until you withdraw them to your bank account, so you will be able to use those funds to purchase with. However, this has no bearing on tax - your sales will have been reported whether you choose to withdraw the funds to your bank account or spend them. HMRC will still know what payments you've received.