Right gents, I'm trying to do half a dozen things at once at the moment, so forgive me if my reply is brief.

As far as I'm aware, the rules on offering to buy from the back of a valuation thread also applies to PM offers.

However, asking to be informed when the item is available to buy seems like a bit of a grey area and I'd like to hear the thoughts of the other mods before giving a definite answer.