You could just list it on the major auction site, and eat the fees (which would come to about $500 between the listing and PayPal fees). This might fetch the highest price, and the highest overall value - it's a big marketplace.
But, there are no shortage of horror stories with auction buyers - the auction rules are skewed towards protecting the buyer and not the seller - and this privileged position is often abused.
On balance, if I were you I would sell the watch here on SCWF (largest Seiko/Japanese-specific site) or on Watchuseek (largest overall watch enthusiast site). I'd only sell it to a member with some standing (by way of references and reputation/activity).
To do that, you need to come up with a valuation - by looking at comparable items which have sold in the past. Unfortunately this is a little hard for rare items like the 6215-7000.
It's against most watch sites' terms of service to provide valuations publicly, but if you post some pictures and ask for feedback via private message, I'm sure some users will offer opinions.
From there, you can offer the item for sale at what you consider a handsome price, with an optional or-best-offer/OBO to cover the case where you overpriced the item and don't get any full-value takers.
The most important bit is that if you wan't to get good value, you need to take your time to develop an understanding of what good value is. Or... you can ask a crazy price OBO and see who bites.
Good luck with the sale - I can't wait to see pics!
I agree with "estrickland" and would also add that simply monitoring the auction sites such as eb$y will eventually give you some idea as to what people are offering and paying for this particular watch. It takes patience, but it will pay benefits in the long run, whether you are selling or buying.
:welcome: to SCWF!