I've found that buying from fellow TB'ers is a pretty safe bet. Look for someone who has a history of positive feedbacks and/or whose posts show more experience, brains, and maturity than posts that only ask things like "Yo dawgz, I'm tHe mOst BruAl fReakin' baZz plaYer on thIs p*Ssy weB site. WhAt's tHe moSt bad-freakin'-aZz bAss for mEtaL?!?!" If you can buy a bass from someone whose posts clearly show they have pride in themselves, their craft, their gear, and their reputation, you'll usually make out *very* well buying from the For Sale forum.
Also, it's a good idea to spend some time in a store just trying out everything you can that's a little above your price range (remember, you're buying used so it's OK to aim higher up the food chain). It won't take too long to start getting a feel for the kinds of tones you like, what kind of necks and bodies feel best to you, does scale length matter to you, do you have a preference for active or passive basses, is there a particular look you want to go for (or avoid), etc. With those kinds of questions answered you can make a pretty safe and reasonable decision. Plus, if you find out that bass really doesn't work for you, you can sell it here for what you paid for it or, better still, trade it for something else on your list.
Believe me, I know it's awkward going into the stores as a beginner. I did the same thing when I started many years ago (at age 37!). However, I told sales people that I'm just starting out and I want to try things out just to learn before I buy. Most were happy to leave me alone to do my own research and were also happy to help when I did go to them with questions. It should be easy to tell if the salesperson is really trying to help you or just push you into SOME kind of purchase just to get a commission. If you feel pressured or feel like you're being fed a bunch of BS, either go to another store or go back to this one another time and try to find a different salesperson.