Reputation,  product quality, price, shipping time, customer service.  In that order. 
I'm new here.  Not a bad board but a lot of people shilling for certain sources/products.  It can be hard to tell who's not getting paid to hype up sources.  I find evolutionary.org has a better review/comments section/system set up.  A longer track history.  A lot of the sources here are advertised on Eroids as well so I can tell this site is legit.  Eroids is king tho.  Before pulling the trigger on a source here, I'd research it a bit on eroids