I recommend expireddomains (dot) net for buying domain, where you can sort things as per your requirements like (removing fake PR, setting backlinks greater than 500, 1k, 20k as you wish, etc..)
Follow the steps for buying expired domains :
Step 1 :- check its domain age, backlinks, then also make sure whether the domain you chosen got real Page rank or fake by using following PR checker tools
seomastering (dot) com
cekpr (dot) com
rankchecker (dot) com
Step 2 : Google it the domain you chosen, site:yourdomain.com to make sure whether the previous domain owner scammed or anything
Step 3 : According to the recent updates on EMD domains, it will be more risk. So avoid buying Exact match domains
Regards,
Bart