Not sure if this would be helpful to you, but I generally not interested at all at buying used products, unless they are priced at 1/2 price or less. Besides depreciation of value there issues of wear, in most cases lack of original warranty and no free shipping.
Bravus 10d is nice enough sub, but absolutely gets destroyed perfomance wise by both SVS PB1000 and HSU vtf-1 mk2 and both cost less.
I recommend you to sell them locally if possible. Imo: Price them between 300 and 350 each and see how it goes. They still could be good supplement for a PC based 2.1 music system, but I think too small and underpowered to be used in HT system
Not sure of the rule of thumb for subwoofers, but I have never sold any of my used speakers for 50% or lower. I sold my Paradigm 100s on Audiogon for 2400, I paid 2900 they were 2 years old. I sold my CC690v4 that was 4 years old for 1050, I paid 1200.
My point being, is I would always check sites like
www.audiogon.com, ebay, even craigslist to get and idea of what people are selling them for. With ebay you can see a past history by checking completed auctions. I have sold a lot of my old stuff through these sites and have found that speakers usually tend to hold their value better than AV electronics. Items that are much less rare to show up on these sites are the center channels and surrounds, which tend to go for a lot more than the 50%, in my case closer to retail. (with my discount when I purchased them)
Older than 4-5 years then usually the 50% might be a good starting point, though I did managed to sell my Paradigm Studio 60v3s for 850 after 6 years of ownership and I paid 1400. More popular brands can be hit or miss, I wouldn't default to 50% or less unless you do your research first. Take in account if you bought them at a discount when you got them as well. I got 25% off from my dealer, so that is usually better than most people get who are 1st time buyers.
I also keep all my boxes, packing materials and a copy of the receipts, which does go a long way in adding a little value to the selling price, not to mention, non smoking house, pristine with 0 cosmetic flaws.
I would avoid selling on ebay if possible, you will get hit with about 15% if fees with paypal