In theory it's simple, in practise maybe less so. Basically the copyright holder should outline the transfer in writing, identifying each/all images (eg subject, place, date) and include 'a consideration' (eg nominal sum of money). Both parties to sign and have both signatures witnessed by someone of status such as a solicitor, should the deal ever be called to account.
That would be watertight but two things spring to mind. How do you identify thousands of images easily? With great difficulty I think. If it is someone's lifetime library, that could be identified simply as 'every picture they've ever taken' and that would work in theory, but unless each image is indelibly marked with the author's name, actually impossible to enforce. I guess thumbnail prints of everything would work, maybe printed 50 to a page of A4 and every page signed over by the copyright holder. That kind of thing.
The other thing is that images can quite easily have two or even multiple copyrights holders, and other people might also have rights in the images, either inherantly/automatically or by way of parallel commercial agreements.
For example, on the multiple copyright thing, anyone who has done Photoshop work on the images could claim rights to that aspect. On a fashion shoot, the model could have commercial rights and moral rights, the clothes designer, the make-up artist, or the hairdresser. On a location shoot, maybe the property owner would have a say, or the owner of that vintage car. Stuff shot pre-1988 probably belongs to the commissioner rather than the photographer anyway. And so it goes on.
If you want watertight, you would have to go through every image, identify it, identify all rights holders invloved, and make sure all their permissions were also in place. You could probably do it on a shoot by shoot/assignment basis, but it would have to be done.
On the other hand, while it might be extremely difficult to draw up a bulletproof document on something as potentially complex as this, there is probably an acceptable compromise. Presumably this collection is currently already being traded commercially by the copyright holder and you just want to continue in the same vein. That shouldn't be so difficult.
However, with a big collection created over time, I think it's unlikely that the author is actually the sole copyright/rights holder of everything and that, technically, they are therefore not in a position to transfer rights they don't own. You would have to go through the collection, make an assessment of the potentially risky areas, make a judgement call, cover things off as best you can, and hope that you've got it mostly right
