when an offer is made what advice can the estate agent proffer?
Your estate agent should be able to advise you on whether the offer is reasonable or not. Isn't that part of the service they provide?
However some caveats apply.
In extreme market conditions, slow or fast moving markets, emotion also comes into play. In these circumstances your agents "enthusiasm" may well colour their judgment.
Unfortunately in England and Wales, you only have the estate agents opinion on what your property is worth. In Scotland sellers have the advantage of knowing what their house is worth before it's marketed (via their Home Report).
In the current market people typically want to achieve more than they will get for their property in a reasonable time.
Your estate agent will offer an opinion on whether the offer is acceptable for the current market. Some would say you're lucky to have an offer at all in the current market (June 2010).
In the absence of a survey whether an offer is good enough is only a matter of opinion based on the available evidence. If you have doubts about the offer you could consider having your own survey completed for peace of mind.
This may be useful in instances where the property is unusual or there are few recent sales to compare to.
Online valuations, free or paid for, are also limited by the availability of comparisions and the extent to which your house is unusual.
The estate agent should also take account of your circumstances and their "feeling" about the local market. We recently sold a house in a fairly weak local market in Derbyshire. We had plenty of offers and no matter how low the agent was keen we considered all of them seriously. I think that's quite revealing.
I hope this is useful.