They are nice to pump the air out of a bottle of wine, not a carboy. If you have a lot of gas when you do this you might think its going to hold but as more gas comes out you will loose the vacum and maybe contaminate your wine. Other DIY methods that work to varying degrees but with much less effeciency are the hook it up to a food vacum bagger, the brake pump, the put it on top of the washing machine while its churning, stir a lot fast etc. Keep these handy, we use them in our bottled wine after opening them and they work very well for preserving the wine.
Keep trolling ebay until a nice vacum pump comes up, its like 1000x more easy to use a real pump than fool around with things that can only draw off a little gas at a time.