Only look at a cocker if you are technically minded, or are happy with sending the marker off every now and then to get it retimed. There's a lot on Cockers that can be "quirky" they are certainly NOT ideal as a starter marker.
And it depends how far you want to go. If you want to get into Tournament paintball then it could be worth picking up a cheap electro-pneumatic to cut your teeth on, Impulse, cheap timmy, cheap angel (LCD or the like).
If you just want to play woodsball, it might be worth picking up a Spyder (electro for ROF preference), Tippman98 (or A5 for ROF preference), or one of the spyder clones (Pirahna, Diablo, Jt Excellerator series). Automag as an outside bet.
Difference really being how much you have to spend.
Whatever happens, if you like the look of something you see advertised somewhere, give the guys on here a shout first...that way you'll know what questions to ask the seller, and what to look out for.
Have Fun