Some lessons we learned in a few months of setting up our store:
Best way to earn a reputation is to offer a good customer service. Be responsive. Answer all your customers' questions. Speak good English. I think that 99% of non-scam transaction problems on DNM are due to laziness and lack of communication. We made customer service a priority when starting up on Agora, and many of our customers returned more than once. It's even more basic than "marketing" or "psychology". People want to be treated with respect, and (not surprisingly) that includes people who are anonymously buying drugs online. Respect your customers, and you will earn a good reputation.
Another thing I should mention is be transparent with your customers. Someone complained to us once that a couple of pills broke during shipment. We didn't try to cover our asses. Instead we improved our packaging, so now there is a double layer of protection.
Other customers had doubts about the color of some of our Adderall. We did our best to explain how Adderall is manufactured, and provided information links to the particular pill that they purchased, verifying its authenticity. We also test all our Adderall with standard reagent tests, because there is some fake Adderall out there and we can always provide this information to the customer if they have any doubts.
NEVER ask for FE. I don't understand why this is even an issue. Buyers should simply never accept these terms. And any vendor that requires FE (at least for domestic shipping) is very suspicious indeed. The vendor has all the tracking information, as well as his previous reputation to uphold him in the case of a dispute. Therefore the vendor has nothing to lose, and the buyer has everything to lose, in case one party scams the other. If you are an honest vendor then there is NO REASON why you would ever require FE. (And indeed this is against AlphaBay policy for vendors not meeting a certain amount of sales revenue.)
Finally, be firm with your security practices! Insist on a full name and real street address. Never give our tracking information. Customers often don't know what is for their own good, and although you might lose a couple of pissed-off buyers, you will ensure the safety and anonymity of everyone else, as well as cover your own ass.