Dallas Artist had some great advice in another forum;"has anyone sold anything on eBay?". He seems to know his stuff pretty well. My selling ART background is limited. But, my SELLING background is pretty damn good. In the car business a repeat buyer was always good for a little more money than last time..You had built some trust with them, they saw value in some part of the transaction. Maybe the product, maybe the rapport between you, the service...whatever. The fact that they came back for more is good!
What did they buy your pieces for at auction? Was it a steal? Was it what you would want if you sold it on your own? If not, explain that to them.."listen, my work usually sells for this, $X.00. At a gallery it would sell for $X.00"...However you feel comfortable wording it...(Don't act weak, nervous or uncomfortable, act casual, professional and in control. If you hesitate, there are people out there who will notice and see that as an opportunity to manipulate you.)...Also how does this work compare in quality to the other pieces that they purchased from you? Is it better? noticeably so? On the same par? If it warrants more money, charge 'em more money. In your mind set your price a little higher than you want to get, but don't gouge, It can backfire fast. If there is no gallery involved let the customer know that. Remind them,"at a gallery, my work sells for $X.00. But since i don't have to pay them a commission I'll give it to you for $ Y.00"... now they're aware that they are already getting a deal. Some may try to dicker more ... a simple," no, I can't do that."will usually work. If they counter with a fair price...Take it ! BUT...let them know " I'll give it to you at that price only because you have several of my pieces and I'd like to have you as one of my Collectors." Make sure that you say "One Of" and make it sound like a privilege for them. Don't sound too snotty, sound professional like you do this every day, Make sure that you are grateful for their business, but don't kiss their feet. If you're too grateful they'll think they overpaid.
Its okay to practice in front of the mirror first. In fact do that, find wording that feels comfortable and easy, but sincere. Even if you did practice in front of the mirror, you don't want to sound like you did. That was really more on the selling end than the pricing end. but if you spend all the time on figuring out a price and they say No to it, Then what?
I hope that might help!