xelloss is right about patience. Sometimes you 
may see a really nice cel that you want on JJ, 
but for more of a price than you should really 
spend. Many times, if you can wait just a month 
or two, you can find that same cel for a better 
price and avoid buyers regret. I usually get my 
cels from either eBay or trusted anime websites 
and galleries. If you are buying from a gallery, 
post an email here and try to get some feedback 
on the person you are buying from. Usually, other 
collectors are more than happy to let you know if 
the site is on the level. I personally only use 
YJ for wishlist cels or dream cels, unless the 
cel is really cheap and I cannot avoid the 
temptation. Some series can be found more often 
in YJ than on eBay, usch as Evangelion and X. 
Good luck and let us know what series you are 
looking to collect. Maybe someone here can help 
you find what you are looking for.