Selling on consignment worked well for me. I sold a student-quality 7/8 cello a couple years back on consignment through a local shop. The commission was 30%; however, they were able to sell it for about 30% more than I would have asked if I were selling it privately. I was pleasantly surprised, particularly since I thought a 7/8 cello might languish in the shop forever.
My husband has sold several instruments through a private dealer without a storefront. Usually he determines in advance what amount he wants to get out of the instrument, and then the dealer figures in the commission and prices it accordingly. The dealer's fee is usually 20%. Again, it's worked out well and we've always been able to get the price we wanted. It's also worth it, IMO, to have someone else worry about showing the instrument, shipping it out on approval, and the like.