Help - Adcom GFA-545 works in mono only

It's broken. You should contact the seller, tell him it is broken and get your money back.

Ok, thanks for the replies.

It's a little weird that the signal is coming out as mono from both sides, if one side would be dead.
Sounds more like somebody did a rewiring job or something... or is that a wrong assumption?

I would possibly suspect 'a' previous owner blew one side out, rewired it to run both speakers off the one working side. and then passed it off as a working amp.
So you have half an amp doing the job of both.
Whomever sold it to you perhaps? never knew it is messed up. Your best choice is to return it while you can. If you cannot return it. i would think it will cost you as much or more to fix it, than to find another used one.
Next time make certain you have a recourse, if you are stuck with a lemon. You may feel you want to 'save' your purchase, but if you can return it, do so. It is not as advertised, and if they only guaranteed to be not DOA, it IS DOA, in one channel. If it was sold "as is", then i think you have a partially useful doorstop.
This is, of course only a guess as to what the problem is. Since only paying someone $100. just to look at it, and is not even a start at actually fixing it...
Sorry.

don't look back, not worth it, very unlikely you'll get satisfaction

Baloney...if it doesn't work in stereo GET YER MONEY BACK! Ebay (and Paypal) are supposed to protect you...and I bet the seller would prefer a return over being blasted with a bad review.