Everything boardgames
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Easy. Build another bookshelf.
Or, you could try trading at a local meet-up. You won't get what you put into it initially, but there are sometimes some decent trades. It really depends on what other skills your wife has, which will determine her trade in value.
My most recent conclusion is that most of the time it is not worth the time to sell stuff used. So either keep it or donate it. I do like the trade idea.
So I second the second bookshelf idea. It is ok to buy one. Also ok to tell your wife that's what you're going to do because you also live in your house.
This is kind of an insulting comment, if the OP could have had the means to do this maybe they would have. They just wanted to know a good selling method and you tell them something they didn’t ask for.
I've had success selling/trading them over FB marketplace and Offerup.
Don't expect to get what you've put into the games. Used is used no matter what and people rightly expect a good discount because of it.
I found that selling the game on "public" spaces like ebay or local variants will often be sold at a heavy loss. Unless you are selling something not so common, expect many offers for the same game, often 60% or more reduced. At the moment we still have the space and so decided to hold onto them for now. We have offloaded some of the less played games to our house in the village and only can play those whenever we are there.
In general many "popular" board games can be bought brand new at 25%-50% off. So if you aren't at least competing in that area or somehow offering something greater then the offers you received were reasonable.
But really I think you simply lacked any incentive to actually sell any of your games in the first place.
We were on the fence. We moved to another city and wanted to shed as much as possible. But in the end it was not worth the trouble. Selling a game for 10-15 euros is not really time well invested. At least at the time ;)
I list them on Kijiji (a Canadian local marketplace website). You typically have to wait a while and relist, but they eventually sell.
Facebook Marketplace, to dedicated groups. I don't like using Facebook, but it's selling pretty quickly everytime.
Otherwise, there's a FLGS that does used games sales every season. You bring your games and if they sell, you get credits to use at the store.
I use board game geek marketplace. Then use pirate ship to quote and calculate shipping. I’ve sold a dozen games that way, as long as you list the game cheaper than others you’ll get messages about them.
This is my go to approach to sell games:
As a rule of thumb I usually discount 30% from their retail price. Depending on the status of the game it may be cheaper or more expensive.
Also be open to trades, sometimes you can get a game that you wanted to try or that's easier to sell.
If you want to sell them ASAP bundle everything together and offer them at a big discount.
I just do eBay tbh. Having the shipping all sorted for you makes it a breeze. Not the best bang for buck but I prefer the convenience over having to deal with constant haggling on other market places.
@Beardsley Interesting Question. I would support the "extend the storage" solution.
I made no good expierience with local trades, but at our capital boardgames trade fair, in the used corner, I did some acceptable sells. Also Ebay gave me some deals....