1. Don't mess with the building if you can avoid it
Building codes are updated all the time and while it’s fine to leave an existing space as-is, once you alter the structure in any way then you’ve got to bring everything up to code. Say you move an electrical outlet—that £80 job could quickly turn into £3000 if the inspector signing off on that change notices some other discrepancy that makes him ask you to open up the panel, at which point he could find all manner of expensive issues. The moral: leave it be or be prepared to overhaul everything at great expense. The other moral: this totally sucks!
2. Hire good people (with little experience)
Your front-of-the-house will work mainly for tips, which will likely prove sparse before you get steady customer flow. For this reason, seasoned vets will typically steer clear of new joints. The key is to select novice workers whose potential you can nurture, like a benevolent restaurant professor.
3. Prepare to get grilled by critics (and actual grills, probably)
Eons ago, restaurant critics would wait an entire month before reviewing your place, which is basically just long enough to figure out how the oven works, but still, it was at least reasonably considerate. Now you're lucky if they let you open your place before lodging the first reviews. As with everything else in life, learn what you can from the criticism and pretend the rest of it never even happened.
4. Don’t be a diva about your "vision"
At the end of the day a restaurant is a business and not a modern staging of Wagner's Ring Trilogy, which is probably an opera. If your customers are consistently asking for something, try and provide it. If people are asking forenchiladas suizas at your Mexican restaurant, those patrons aren’t just asking to be fed, they’re really saying, “I would very much enjoy giving you more money. Why are you making it hard for me to give you money? Look at all this money I have to give you". You should listen.
5. Don’t engage the haters
Part of working in the hospitality industry means dealing with people who are impossible to please. In the food biz, you’ve got to do what you can, when you can for those problem customers. But don’t bow to the will of every complainer. When it comes to customer feedback, it's best to look at what trends emerge rather than to worry about a solitary gripe (if multiple people are leaving online reviews with "Uh, the kitchen was on fire", maybe your kitchen is on fire).
6. Things break. Be prepared.
A lot of the restaurants listed on eBay are fully furnished & equipped, but even new things fall apart, because they're being put under unimaginable strain by your hopefully very busy establishment. Think you'll be using a particular piece of equipment 1000 times a year? Do the math and it's probably 20,000. Then again if you could do math you'd never open a restaurant!
7. Make sure you have more than enough dough
There's a pervasive myth that nine out of ten restaurants close within the first year — it's actually closer to one out of four, which isn't as terrible but it isn't rosy. To better your odds, make sure you've got a lot more money than you think you need, and be prepared to wait a long time to turn a profit. Make sure your investors know that too or you'll make them unhappy or litigious or possibly leg-breaky.
8. Location matters
Where is the restaurant? Is it a high-traffic area? What's the rent like? If it's far-flung, do I realistically think I can attract customers? Where are my keys? These are essential questions, because the same service offered in different locations can easily end with one closing and one thriving (also, it's important to know where your keys are).
Buying a restaurant that's already up-and-running (or one that used to be) has the potential to make things easier. But the plain fact remains: it's going to be hard. As of 2014, there were 990,000 licensed restaurants in the US. If there's nearly a million of anything, it's not impossible to pull off, but there's going to be competition and struggle, so prepare your body and soul for the ride of your life... provided by "ride" you mean "shutting yourself away in this restaurant 16-hours a day for the rest of the foreseeable future. Don't worry, the smiling faces and disturbingly extended bellies make it all worth it.