Whilst you can read a disc which has been over-written, this shouldn't be a problem:
(i) The more times you over write, the less easy it will be to recover the original data. If this over writing has been with random data (strictly speaking, pseudo-random, but that's probably close enough), it will be very hard to recover the original data. Thirty Five over write cycles should have effectively wiped out anything (and this is all Boot and Nuke does, although I'm not sure how many cycles it goes through).
(ii) To recover information requires very invasive work on the disc, at a minimum you would need to replace the drive electronics. To read the data, in essence there is still some element of the previous write. For example, say you had written a bit as a one, and then a zero, there could still be a slight remnant of the one bit, but as a logical bit (which is what the drive will normally want to read) it would look like a zero. With suitable analysis of the analogue data, you could recover some remnant data.
If you've formatted the entire disc (and presumably will give the buyer the original OS discs, so they can reinstall at their leisure) then I seriously doubt anyone who had capabilities less than a serious forensics laboratory would have any chance of recovering any data. With multiple random overwrites, it's probably beyond even the most advanced techniques.
Of course, governments are very very paranoid about such things. Not only do they want to guarantee that data has been erased, but they want to guarantee that no possibly developments in technology may allow data to be recovered in the future. The only real way to ensure that there is no even slight risk of data coming back from the bit bin, is to mechanically destroy the disc, ie melt it down, or grind it up. This is probably unnecessary to protect your credit card details buried somewhere in the remnants of your web browsers cache, and the odd salacious email.
