I have run a number of silent auctions and I have asked donors to NOT make open ended arrangements. They are fraught with difficulties. You promise one thing and it gets interpretted as another.
I agree with Spiceycat, make the specific arrangement, i.e. a studio in BWV checking in on X and out on X with one T&T on (Day).
Be sure the let them know the rack-rate value, not the value in terms of points, thus they can start the bidding higher and expect more. If you want, you can offer the chance to make minimal changes and specify a cut-off date that gives you the time to cancel and save your points without loseing them.
I instruct all my donors to think it through and remember it is a gift from you to them, thus you do not need to be TOO generous.
If it sells, transfer it. If it doesn't, and there is that chance, cancel it and save your points.
Been there, done that.