IMVHO, a GPS is possibly the very last bit of kit to buy... you may find that you buy a boat with one already installed! Also they can be misleading in the hands of beginners....
Also beware of bidding over the odds on Ebay - some people (I was tempted to type "muppets" - d'oh!) bid up to within a couple of £'s of the new price. Always do a google search on the model name/number, you can get some great "proper" bargains!
I have an old Garmin GPS40 which I don't use any more. I haven't checked it working, but if it's OK I might put it up for sale.