your best bet is to see if someone you know already having a detector will lend you one before you buy it- I decided after my step dad lent me his that I wanted one...
check out any local places that sell them - if you live in FL you have it made (you can try them in the store before you buy.) I have just got an ACE 350 -for about $275 or so- kellyco has really outstanding customer service, they advertise on here so you can check them out- if you go too cheap you'll be unhappy with your purchase (I know I spent like $230 on a bounty hunter and it stopped working after about 5-6 outings), and difference about $100 makes means digging annoying junk or not. BH are a little on the cheap end (no offense if anyone has them and they work for you) - BH customer service never returned my calls, so I'd never do business with them again - that's the main reason I returned their product.
anyhow - Minelab, Fishers and Garrett seem to be what most people use- and their are other options that have great warranties so your best bet is to contact a dealer and ask around- do about a month of research before buying -as I rushed out and regretted buying something that didn't hold up- because I just wanted to get out and find cool stuff.
I don't know that I'd try E-bay for one - electronics are risky - and seems like a lot of people aren't on the level - one suggestion on if your new and green like I am....when your holding your detector practice using it from your shoulder not your elbow, as you will have tennis elbow bad in no time at all!
my first few times out digging up trash at the local park it was cold, I was sick with flu, elbow was hurting and I was finding trash and clad...but it's so fun it beats my best day at work.... lmao.
