I have two Siebert 12.5mm EP's. Each costs $45 +s&h; I will sell for $40 each, including shipping to Con.Us. You may buy just one, of course.
I used the 12.5mm on my 6" dob -- they gave very satisfactory viewing. But the short eye-relief (6mm or 7mm) was what made me decide to let go of it. (The other 12.5mm was supposed to have been a Christmas gift for someone...but I found it he already had a 12mm Lanthanum. So I ended up with 2 of these Siebert babies.) I will take money order, or discuss with you other ways of transaction.
P.S. I have a Siebert 7mm for sale under another entry.