I can understand while genuine old garage signs fetch a lot of money, there aren't that many still around and there are lots of people out there who like to collect them.
But given the danger identified by Dave of there being fakes out there, I am perfectly happy to buy reproductions, they brighten up garage walls and don't cost anywhere near as much.
There are, however, a depressingly large selection of 'modern' signs which are frankly a bit naff - as a 2 minute perusal of the displays by the vendors at the NEC Classic show will demonstrate.