I've dealt in part worn tyres so I'm pretty sure I know how it works. It's all down to the tyre, most tyres are old (usually 3+ years) so peak grip has gone and then they're worn down to around 4-5 mm if you're lucky. Not something for a performance car but for a runabout it would do the trick.
If you're lucky enough to get some decent part worn tyres, the price will reflect that as tyre garages know what it's worth. So you may aswell go for new ones as in the long run they do pay for themselves. Bottom line, not something for a GTI unless you're selling up.