what happens if you're sleeping and bitcoin spikes in price to $20,000 while you had limit sell orders of $18,000? someone will have lifted your ask and what are you gonna do? try and cancel? 
Most platforms have methods built in to deal with this. For example, on HodlHodl you can set a stop loss which will disable your trade if the price fluctuates outside a price range you have set. Or on AgoraDesk you can use floating price so your trade takes places at the price when the offer is completed, not at when it was posted. Or you can just take your offers down when you go to bed.so you want to do a face to face trade with someone and charge them significantly over bitcoin's market price.
If you are the only person offering a service which is in demand, then you can charge a premium for doing so. This is not unique to bitcoin.