I see that many people sell their credits. I'd like to do that, since it would save me a little money. How do I know who is reputable?

I've never found anyone who wasn't reputable.  I've sent Money ORders and they've sent the credits.  Or I've sent an Amazon gift certificate and they've sent the credits.  I've dealt with several on here but one I'd trust above everyone is doc - Winston C. (Crabby-doctor).  He's one on the very top swappers on the site and almost always has credits for sale.

WhenI was new and bought some credit, I looked for someone who had been around the site for a while and had a history of posting in the forums. It's not foolproof and I am sure there are good new members too, but PBS doesn't take scammers lightly so I felt confident that someone who had been active on the site for a while wasn't a scammer.

Just don't 'gift' your credits until you get payment. And you're limited now to gifting 50 credits a month total, so you can't sell more than that.

So far I have bought credits 2 times since joining & they were both very easy transactions & were given to me the same day that I sent payment (I bought through Paypal).  So I've had good luck with it thus far!

I've purchased credits about three or four times now.  One even sent the credits before I paid.  Good thing I am a trustworthy person. 

You are in a better position than the person buying the credits because you shouldn't send them until after you have payment.  It's the people buying credits who are taking a real chance, not the sellers. 

I'd suggest trying to be online around the same time as whomever you agree to buy credits from so that the transaction can happen faster. When I have bought credits in the past, I send the member selling a PM. They respond back with their paypal information. I transfer the money and send a PM that the money was sent and reputable sellers transfer the credits pretty fast (if they are online).

Good luck with it.