What percentage of revenue do the top 10 customers make? Often there is the 80/20 rule, 80% of your revenue comes from 20% of the customers. If the revenue is more concentrated than this, you may have economic dependence on a few key customers. What risks are you going to take over? You can mitigate your rists by having the old owner provide you a vendor take back financing and the purchase price is a formula based on the retentenion of the large customer.
I have seen cases where the revenue from one large customer was generated from the fulfilment of one contract spanning over 2 years. You have to make sure if this is a reoccurring customer or just a one time customer with a special order or contract.