Let's assume this is possible with command blocks, and that the admins/crafty have nothing better to do with their time. Highly unlikely, but I'm willing to make it for sake of argument.

Some problems I see:
Price to purchase a shop

• craftycoins are out of the question, too many already exist on the server
• there's no way you could set the price in diamonds or any other in-game item. Either you'd have too many players trying to buy if the shop is too cheap, or once you make it too expensive (maybe ~32 or more diamond blocks per month) very few players or nobody will use the shop. Perhaps Miz and Neon's shop has enough steady business to still profit at this rate, but pretty much all other shops would fail. Therefore, shop availability would have to be decided with either vote balances or based off vip/vip+/server bill donation rank
• I don't believe spending votes could be stable in the long run. Each player can currently vote 120 votes per month (4 votes daily and about 30 days a month), so if the price was anything under 120 there would be a risk of dozens and dozens of players voting everyday and demanding shops. If the price is much higher (maybe 200 a month), too many players would have shops the first month, but after a few months have passed most people will run out of their vote surplus and the shops will be a ghosttown.
• therefore the final option is donor ranks. With vip you'd have too many shops, so the last option is vip+/server bill for a shop subscription. Although I myself have purchased vip in the past and currently have 1/4 server bill, I don't think it's a good idea for players to be forced to pay for what would be such an integral part of the server. I also think this would anger more people than it would please the people with the addition of player shops

stock of the shop

• I don't see many shops being constantly filled. Except premium items, many trades on the server are done in bulk, so I'd see many shops being sold out quickly. Someone might clear out a shop for glass for example and then leave it unstocked for days. Shop owners would have to constantly check their shops to see if they were sold out, it would be exhausting. Buyers would constantly bother shop owners when the shop would be refilled (at this point both players are online, so why not just sell through the trading post like we've been doing for months?)
• Also items like god tools are often custom made (regarding knockback and fire aspect on swords, or silk touch vs fortune on other tools). Players would need a ton of shop chests to sell different permutations, and it would be infinitely harder to keep stocked than to simply custom make the product.

Location

• there's no convenient place to put the shops within spawn
• placing them too far outside spawn would make it too inconvenient for players to walk just to check (a) if an item is for sale, (b) if it's currently in stock, and (c) what the price is (and even making a trip back to their base to get the correct currency, if necessary)
• i guess it might be possible to put them at the 64k,64k spot the casino and other features are at, but you'd still run the risk of the previous option: if it's too tedious to get to and check shops for the product you want, people are much less likely to buy

tl;dr put more thought into your idea if you want anyone to take it seriously