Basically, it's a contract you need to read, because there's usually a lot of money on the line and you can be tied into a deal with a useless agent. Always, always negotiate the percentage and term down (they'll always pitch high). And use the fact they'll claim 'to have a buyer' – if it's going to be such an easy sale, they'll obviously take a low commission and short tenure, won't they, as it'll be easy money...
If I were to ever move and use an agent again, I'd try the local independent – the ones we dealt with on the vendor's side (because our own agent – KFH – evidently couldn't be bothered) were a local affair. That said, this house had been on the market for eighteen months, so they were pretty desperate to move it, even at £50k off already reduced asking price.
On the plus side, we now have a laughably small (by London standards) mortgage (which only exists because we can only pay 10% off each year without triggering an early repayment). But I did begrudge the >£10k we paid out in agency and conveyancing and other fees for frankly not very much service – and you have to chase every-fucking-thing.