So if you want that prime shop you're going to have to pay the owner for it one way or the other. The market being unoccupied and empty as a whole is more of an issue than shop squatting and I think we should take care of that before fixing this. Otherwise in the end we end up with just more empty shops.
Im conflicted on this whole thread. I agree that shops should follow the already established basic rules. I dont think people should hoard valuable shops but at the same time dont want to limit ways to make money which that is one way to do so. This whole thread is a tough set of choices. I say do away with the market all together and allow for the forming of market towns (i currently am working on one myself). The market is fairly convenient, problem is not the whole market really is. Markets not filled and is like walking through a ghost town if you actually walk through it. I think either dropping the price of a market stall and enforcing the rules of having one or just doing away with the market entirely are better choices, either those or get rid of /market and make it so those with a market have to actually advertise them to make money which would do away with their being prime spots since all markets have a /warp. IMO the market is just poorly designed and has always been, the space in a stall is limited so most of them have the same blocks and similar if not same prices for everything. Market needs a redesign, remove the /market warp, and allow for larger stores or even have different sized stalls for those who want big or small ones and prices to reflect that.
Those are two different problems with two different solutions, though. The overabundance of market stalls is indeed a large problem. However, you'll find that this overabundance of stalls is concentrated in unattractive locations. This suggestion concerns market stalls in attractive locations. Nobody rents stalls in unattractive locations, so those stalls are quite irrelevant to the suggestion. The problem is that the stalls on attractive locations are many times not utilized properly, which reduces competition, makes it harder for users to find the shops that sell things they need, and makes it harder/more expensive for users who actually want to maintain a market shop to get one in an attractive location. This suggestion is supposed to tackle that problem. The problem you're citing isn't what the suggestion is about, nor will the suggestion's proposed solutions tackle that problem.
At no point have I meant to imply that what I've said would fix the issue, but it doesn't make sense to me to fix this before that. Suppose we enforce this, though. There will still be shops with the bare minimum trade signs that aren't actually intended to sell things and rather just hold space. People who want these shops will still not have them. People who own shops next to them will still slightly resent them for hurting the reputation of that market. People currently doing this with $100k-$1m+ assets aren't just going to go "oh shucks, I guess that's it", or "oh shucks, I better actually build a shop and keep it up to date". They'll simply add some number of tradesigns, maybe build a sad little storefront if we require it. This won't change things.
The guy wants to sell his shop. No one's buying. His intentions are not to own it but to sell it, and may have cleared it when he expected it to be sold.
While I agree that shops, like the on you linked, do indeed ruin the look of the market, I feel like this rule would be kind of strange to enforce. I'm kind of on the fence.