And if such mangling existed, would it stop a person from manually cutting and pasting from the browser? I know we can't stop the cut-and-paste, but would the mangled stuff then require laborious hand editing to clean up?

Well, I guess as a radical approach you could do something like replace every other space with an "i" or some other narrow character, but put it in a font color that's the same as the background color. (You can't do every space, obviously, because then it won't word wrap.) So it'll look normal, but when you try to copy and paste it, you'll get something like this:

Thisiis aiwonderful product.iYou shouldibuy itiimmediately.

Of course, an actual customer who tried to copy and paste the text (say, to email it to a friend who might be interested in the product) would probably get annoyed by this.

As for detection, what about writing a script to discover some unique "watermark" phrases in your descriptions? Here's what I mean. Let's say your product description is this (ironically, I just grabbed it from a random Yahoo store):

Brushed moleskin knee length skirt. Patch pockets front with flirty 9" back center slit. Coco exposed stitching. Zip fly, belt loops. Enitre length of size Medium: 24". Stretchy, light-weight 96% Cotton, 4% Spandex. Hand wash cold, hang dry. Made in the USA.

Use LWP (actually, Google frowns on you doing this sort of thing programatically, so let's assume you get a Google API key and do it all nice and proper) to search Google for each three-word phrase in succession: "brushed moleskin knee", "moleskin knee length", "knee length skirt", "length skirt patch", etc. You'd probably want to skip over any words shorter than 4 letters, because they're less likely to be part of unique phrases.

Keep track of which phrases return zero results (use -site:mysite.com in the query to omit pages from your own site). Then a few weeks later, search for those phrases again. If you find any results, maybe you've got your plagiarist...

Cheers,
--Kevin