Why would you go for tender?
Well you can fix a set date for bids, you have a fixed term. It's confidential so it can hype the market up a bit more as you don't know who you're bidding against in a private tender situation. For a period of 6 weeks normally you have extreme hype and then once the tender is finished, hopefully you have a purchaser who will close within a set period.
For a tender, you need a good solicitor who will prepare contracts - which most likely will be requested in advance by prospective purchasers. This means you can set a closing date if you want to get rid of the property within a set time.
What makes it different to an auction?
It's a confidential and closed bidding scenario. For the vendor, it means that they can pick and choose the best bid, which might not necessarily be the top one. Factors which would influence the decision would include ability to finance and suitability for the site.
Why would you go for a private treaty sale for a development site?
To test the market in the case of a development site. Normally an estate agent would offer it to the market by private treaty & then bring it to "best bids" scenario which is treated almost the same as private tender mentioned above. Tenders can intimidate some purchasers and this allows for an all inclusive marketing campaign to net the most interest and then bring the most interested parties to bid against one another in a private situation.
Fees?
No idea whatsoever - but if you do agree fees, agree an incentivised fee. That way the Estate Agent will work for you, and themselves much harder!
NB
I'm not an estate agent, this is just my interpretation!