This is why real estate in this country is messed up. The person advising you has a stake in making more money if you pay more. So your estate agent will try and sell your house for more but he may also want a quick sale, so you can never be sure if he has your best interests at heart. As a buyer you can also never be sure. Thats why you never, ever go sole mandate. Get 3 estate agents to do independent valuations on your house. If you want to go even further get a bank valuator to do a valuation and give you the report. They can be a bit expensive tho, but at least you know they're not motivated by the sale.
Ask your estate agent how he got to his number. Have a look at the house, and have a look at what houses in the area sold for and what they're selling for. Go to some open days at other houses in your area and see how they compare to your house. If your house has features that the other houses in your road don't have then I add on an appropriate amount and visa versa. Take all that into account and set a price. Add a little on and make it known to the agents that you're open to a little negotiation.
As to what to offer it depends on the interest in the house. If its been on the market for a long time then its either a sign that the house is overpriced or that there are other issues. I would offer quite a bit less and again let it be known I'm open to negotiation. If there is a lot of interest or its new on the market then you need to decide if its the house you want then you need to make the offer you can afford and go with that.
When I bought the house I have now, we made 6 (not counting the houses that had offers before us) offers on houses before we got this one because I only offer what I think its worth, taking into account what I will need to spend to get it to where I need it to be liveable. 4 of those where outright rejections, insisting on their list price. the other two came back with counter offers (which we didn't accept). Our current house we offered R200k lest than list (it was in BAAAD shape) and they countered with R100k less than list and two months free occupation to renovate which we accepted.