Everyone has their definition of "considerable" amount, but if PayPal is in play, then the amount is likely under 5-figures...
I can certainly understand if you don't want to get more specific about the money involved, but my take of the situation is that the buyer either got second thoughts / is over-extended (it happens even to the best of buyers) ... OR ... the buyer is out to scam you...
On an aside, PayPal in all forms is revocable - what happens if PayPal comes back on Sedo ... it wouldn't surprise me if Sedo leaves the seller left hanging ... not saying I'd blame them, but then that's not a true escrow either ... rambling on ... I just can't see how Sedo could ever accept PayPal for escrow payment for more than a say a few thousand, since all PayPal related transactions, including echeck, etc, are revocable to varying extents.
Since Sedo has been no help - and it appears you know who the buyer is ... drop Sedo and use Moniker or heck, even escrow.com - be sure the funding is via non-revocable bank wire only (bank to bank transfers, like ACH here in the states, are often revocable - one must be sure funding is actually via wire) ... Escrow.com and I'd imagine Moniker will only accept non-revocable funds for large escrows, but one should always be sure before initiating transfer ...
Otherwise, you may find yourself with no money and no domain.
In short, it's sometimes better to let sales go then lose it all - if there are other decent offers, wait awhile, work the sale a bit, and you'll likely get the same amount or even more from a legit, more responsive buyer.
Ron