In general, I would warn against that TBH. Not just because I personally don't like E-Bay but mainly because there have been reported issues with Sigma lenses involving bad copies and where do people tend to get rid of stuff like that with fewer potential repercussions? If you can find one close enough to home to do the final deal face to face after inspecting the lens and taking some test shots, fine but as a distance deal, personally I wouldn't.
that is true but that lens is shockingly bad , my mate andrew owned one briefly before he got shot of it
as has been said something like a 150-500 is the way forward
but even a digiscoping set up and a decent scope will give better IQ than the opteka - the only long lens ive encountered that was worse was the cosina 100-400 (that was on par with trying to use a milk bottle)
Are you American based or UK based?