I've had offers from people in your position when I've sold my house - TBH I decline their offer and wait for someone else to come along. At the very least I'd say thanks, will bear it in mind, but would only take my house off the market once you are in a position to proceed i.e have a buyer
So yes, you can do it, but it may not be leapt upon with open arms.
Well it depends. If you are confident that your property will sell quickly, or if you are not dependent on the sale to make the purchase, I suppose it might be reasonable.
In the past when people have made offers on houses we've sold when they aren't actually in a position to buy, I've usually just thanked them for their interest and explained that we will hold out for someone who IS in a position to buy.