Opinions wanted: Cheap rimary mirrors on ebay

Hi there,

I feel sure that this has been asked before, but I DAGS
and didn't really find anything about it.

I'm wondering if anyone has experience with the inexesive
primary mirrors available on ebay. My suspicion is that
you get what you pay for, but I'm wondering how they
would be for building my first Dob scope. I need to
keep it cheap.

I suspect these won't create a fantastic scope, but I'm
interested in something to start with (before investing
a lot of money.) I also enjoy building things like this.

I'm considering the 4.5" spherical or 6" parabolic mirrors
that are listed with the specs below (I know the conversions
dont really add up, this is directly from the listings):

4.5" spherical mirror (~$20 including shipping)---
Clear aperture 4.4 inches = 122mm
focal length 35.5 inches = 902mm
Focal ratio f/8
Mirror thickness 0.5625inches = 14.3mm
Thickness/diameter ratio 1:7.78
Surface Accuracy = 1/8

6" parabolic mirror (~$35 including shipping)---
Coating = al+sio
Clear aperture 6inches = 152mm
Focal length 29.38inches = 746mm
Focal ratio f/5
Mirror thickness 0.625inches = 15.9mm
Thickness/diameter ratio 1:9.6
Surface Accuracy = 1/4

I'm very interested with first hand experience with these
mirrors, but I'd also like any other general opinions anyone
has.

Thanks.