1. What is the best way to take offers?
I've made offers on some items, and here's a few things that I like to see:
A) Post the ending date and time and try to stick to it. It's not unusual for things to run a bit over the stated ending time if you've got two or more bidders fighting over it, but avoid leaving things open ended. A lot of people try to "snipe" on these things, and you'll probably get the most activity on the last day, so make sure you let people know when that is.
B) A bidding war that overshoots the official ending time is annoying to the bidders, but don't let it stretch out too long. No bidder likes to be strung along for days. And if anyone who wasn't involved in the original bidding war tries to jump in after the deadline and starts bidding, I always considered very bad form to accept their bids. And once you tell someone they won, it's REALLY bad form to accept any other offers for the item. I had one seller tell me I had officially won, got my address to figure out shipping, and then a few hours later tell me there's a brand new person interested in the item(they "missed" the auction that had ended two days before) that I had been waiting to pay for, and now if I wanted the item, it would cost me $100 more. To say I was angry would have been a gross understatement.
C) Choose a location to receive your offers(most likely PM because that's what most people will do anyway), and use that to keep the order of bidders organized. People will be posting in the thread, PMing you, and probably emailing you if you post your email address. It could get really confusing who bid what amount first if you don't try to keep things all in once place. You will have to watch the thread though to make sure nobody's posting offers there.
2. Should I put what the current offer is?
Yes, it's considerate to bidders, and it'll cut down on bids from bidders who wouldn't pay enough to win anyway. I also like to have the seller contact me if I get outbid. I usually end up bidding more if the seller is willing to let me know if I'm outbid or not.
3. Should I set a starting price or would it be ok to wait and see what my first offer is?
If you have a price in mind, you'll probably want to put a starting price of some sort.
4. If the pony doesn't make what I hoped for can I refuse to sell, is it ok to put that in the terms on the for sale page?
That's fine as long as the fact that you might not sell if you don't get the price you want is in the original thread.