Although it's not necessarily the best practice as far as the quality of the end product is concerned, the reality is that if you want to be competitive at all shopping beats they must be solid mixes and be "mastered". I put that it quotes because most hip-hop people have twisted ideas of what mastering is. In realilty, you should have a hot mix, then slap a limiter on it (the 'mastering' part) and call it a day. There is no way on earth you are going to compete with me and all the other bozos if your beats aren't at the same volume as commercial CDs. You don't need to go crazy making them louder than everyone else, just in the same ballpark.
As for what happens when you sell a beat... Generally speaking, if it's a major label artist, or a smart unsigned/indie artist, they will also pay for the tracked out beat. however, nobody doing a mixtape (even major label artists) are going to bother with tracked out beats. As well, a lot of cash-strapped artists (even indies) will often just roll with the 2track. Sometimes for cash reasons, sometimes out of stupidity. In these cases the limiting and 2buss compression with hurt the end product, but whatchagondo? You'd be suprised how many major label projects are just vox on a 2track because the producer can't find the files, so they just roll with it anyway.
Last, you must understand that when a major label artist likes your beat, they are going to track over it. Once they like what they've done, THEN they will buy the track from you. So if it doesn't sound good as a 2track, then it's an uphill battle. If you need mastering to make your 2track sound 'better', then do it (although that's a BAD situation to be in!). But regardless, you do kind of need to put a limiter on it for volume just for shopping purposes.