A quick review on GPS receivers as this has become a very popular item on ebay.
There are many different kinds available, therefore many aspects of the product to look out for.
As you are most likley aware, the general usage of these items are for satnav systems, which means that this device will need to have some sort of connection with the another device, namely a PDA preloaded with mapping software, so bluetooth v2 is a key advantage.
Older models still use bluetooth v1.1, looking out for a unit that supports bluetooth v2 is key.

Number of satillties, this varies with every unit, the more the better, as it connects to more "moons" triangling your location more accuratly. However, its reported that there are only 13 sats visible over london [i dont mean visible to us, but the units] thus potentially rendering many 20+sats units useless.
with note to Tomtom, it is satisfied with just 3 sats.
But does mean that the more sats it can handle the better the unit is? not always.
many on ebay now advertise a massive 32 sat capability, but this isnt always paramount. i for one, am more interested in its acquistion time [how long it takes to pick up a strong signal from a sat] this is shown as cold start on many listings. so look out for a low cold start time.

Another more techincal aspect to look out for when pruchasing a gps unit, is the processing chip inside the device.
there are many companies who develop these chips, popular ones are chips that lead with a BT, like the BT-77 which is a 16channel unit, another more popular chip is the SiRF, now in its 3rd generation the SiRF III, a 20chan/sat unit.

To end, as im out of time, after testing many units i believe the Holux GPSlim SiRF III unit rates higher then rest.
As its a 20 parraell  sats is suffienct for any mapping tool, its vey quick cold start time, its extremely small size, and the package contents isnt bad too.

good luck, and i hope this little review helps

nb: i havent spell checked or proof read, sorry im out of time.