As an FO with a B737 rating and just under 500hrs on type who can't get a job I think you'd be making a big mistake buying a rating. As 4star said there are far too many of us about and the vast majority will be more experienced.
I did look at buying time on type through eagle jet to try and bump myself up the ladder a bit. However a friend at a major airline asked their head of recruitment if he thought it was a good idea and the answer came back as a resounding no. He went as far as to say that because it was paid for some airlines might not even consider it 'airline' flying.
Just my opinion from my own experiences and information others have given me.
Good luck with the job hunt.