Saying you'll either use a traditional estate agent WITHOUT making an online listing, or use an online estate agent, is the worst of both worlds.
An online estate agent may be cheaper, but IME it's a complete waste of money; judging from our house-hunting experiences this year, you'll end up caving in after 6-8 weeks and going with a proper estate agent. I've no experience with Purple Bricks, but the online estate agent we viewed one house through was awful: never picked up the phone, didn't even have voicemail, didn't contact us for feedback, was barely interested when we contacted him for feedback (we liked the house, but it was very overpriced; 8 months later, it's dropped down to what we would have offered, and has sold via a traditional agent. All comparable properties nearby sold months ago).
In contrast, the high street estate agent who dealt with the house we eventually bought really did earn their commission, doing tons of work to keep a complicated chain together, liaising with solicitors, other agents up-chain and vendors. The initial marketing is just the start of it.
The online listing is vital, though. We did get lots of emails and calls about hot new properties from various estate agents once we'd started doing viewings, but we mostly ignored them since they weren't tailored to us at all: 'look at this great studio in postcode Y!' when we were looking for a 3-bed in postcode X', that type of thing. So without an online listing, your property may disappear under a pile of annoying marketing calls and spam.