Oliver, I'm not sure that there are a lot of selling secrets, other than:
1. It helps to be a "known guy" on a major forum or within a network of collectors
2. Collectors' Market here on PuristS has quite effective reach for mid- to upper-level pieces (it's a thin market, but we attract those kinds of folks here)
3. Do your research on what comparable pieces have been selling for, and start your pricing perhaps a bit optimistically, but not crazy high
4. Good (and in this context I mean clear) photos do help
5. Respond quickly to all inquiries
6. Don't be afraid to say no to lowballers, but establish your "best price" in your own mind and seriously consider all offers above that level
7. Buy the right way in the first place (that is, concentrate your purchases of new watches at a few ADs to get maximum discounts on every new piece you buy, and buy other pieces used from reputable buyers at depreciated values) to keep your acquisition cost down
8. Buy the right things in the first place (not necessarily being a slave to what other people like with an eye toward the re-sale market, but understanding that there are some brands and watches that are going to be "fun" items that will depreciate vs. "investment" items that will likely hold most of their value)
9. Take care of your watches. Being able to say, honestly, that your piece is Mint, and having a reputation for telling the truth about condition, are two of the best sales aids
Guess there were more than I thought!
Best,
Gary