When it comes to fielding offers, there isnt much to it. If you get an offer you like, its pretty easy to hit the accept button and move on. The question I have always pondered stems more from when it is a good time to counter.
A lot of the time, when it comes to taking offers on my best cards, you walk away just shaking your head like what was that guy thinking? I have had experienced bunters offer me trades on sigs with 9 base cards. That just isnt worth the time to counter, as its clear that the original offer is so low that there is no reason to try to start the negotiating process.
There are times when you can see an offer is just a bit off, the collection score of the trader looks like there might be something else worth checking out. When you get these types of offers, go into counter mode, and sort by rarity. It will put the rarest cards at the top of the list and easily show you the best they have. Move down from there. If you find something REASONABLE that you like, shoot the offer back with the new trade.
Additionally, under sort, you can tell the menu to only display cards you need, which can be really helpful in finding those pesky cards you dont have.
Lastly, if someone makes an offer on a card I have, I play the negotiation at the beginning for them to overpay slightly. They want the card, so they may have no problem giving me JUST a bit more than its worth. If that doesnt work, they can always counter back. Just dont go nuts unless you are trying to make a point.