First thing you will need to see is the accounting records of the account. Next thing is to figure a worth, street value is typucally 5% of annual net.
You will have to meet with the account and see if you meet eye-to-eye. What if you hate the client and the net is lower than what you are used to.
You will also need to get a non-compete drawn up to prevent the company that sold the account to you from going in and doing the work while you are there.
I'd say for the average detailing business, all of this work is out the ball park. The legal work is in the $15,000 range to properly orchestrate an account sale. And who is to say the compnay you are buying will keep you?
Buying an account is way too risky for a "skilled labor" type business. If the guy is looking for $500 or something that won't matter to you if you lose, then roll the dice. If this is a big volume account, you will need to do it properly.
Maybe approach the guy selling the account with a latent finders fee. Meaning, meet with the account as a referral of him and actually do a certain volume of work for a specified period of time, the seller is entitled to some payment if the relationship works out. (10 cars over 30 days nets the seller $1000.00. 20 cars of 60 days nets the seller $2500 and the contract is complete)
I sub contract through many different companies, I always start off with a non-compete.