My Take On This
Its all a matter of rights. You as creator of the beat have rights on the beat, and it is also your responsibilty to clear any samples used. As their makers have rights too. When you sell a beat you really should have a contract that states what rights, ie full rights(exclusive), or usage rights(non-exclusive) each has its own price bracket and own benefits. If you sell a beat with non exclusive rights, you maintain the copyright of the track and can resell the non exclusive rights to whoever you wish. exclusive rights can be around £1000/$2000 and non exclusive as little as £25/$50. And then theres a matter of royalty payments, thats why a contract is important, if your track blows up and there is no contract, its a poor state of affairs to be in.