I don't think those models are particularly valuable.
But hard to say, on my local 2nd hand sites I don't really find any for sale.
Looking on Ebay, very few for sale at the moment, when searching for "sold" items in the last few months I find "for parts/non working" G3's sold for 65€ and working ones in the 115€ to 230€ range. And that was all in the US.
They obviously aren't "usable" computers for daily work anymore, so they become sort of collectible items, and there the value would be rather in complete sets, with the time correct monitor (which would be the "Blueberry" studio display, crt or LCD).
I can't really recall if the powermacs were sold with color matching keyboards like for example the iMac G3's in "fruit" colors. If so, a matching set would also add to the price.
The iMac G3's are an example of this, pristine complete sets can fetch sveral hundreds of euro's, but if just the iMac without the matching peripherals you'd be lucky to get 100€, if it gets sold at all.
Seeing on how few there are for sale, I'd say put it up and be patient, you might get lucky and fetch a collector who is missing just that model.
I would try Ebay, and set a relatively high fixed price with an additional "best offer" possibility, and go from there.
Or any other 2ndhand platform which offers that way of selling (fixed price with possibility of offers).
