It's tough for me to say, since I haven't kept up with WCs much lately. But based on what I know, it's a gamble. I've converted maps and gotten a codestone, a couple of relatively inexpensive PBs (Silver and Brown), a Red PPPB, and even a Maraquan PB. But this was before they made big changes to the rules, changed the prizes, and basically sucked all the fun out of it for me, and I haven't played regularly since.
I've ranted about how much I hate the new WC system before, so I'll bite my tongue and say that generally, I'd advise against buying an entire map set in the hopes of making a profit, as there's only about a 50/50 chance that'll happen. Now, if all you're interested in is just buying map sets and converting them just to get prizes and/or complete all your sets, or if it would take way too much time and effort to earn pieces yourself, then go ahead and go for it.
At one time I would have said that was the best way to go, but since the pieces don't go straight to your map any more, it's more arduous to complete a map now than it should be, even if you're really good at playing the games. Seriously, I know TNT was just trying to make it fair for everyone, but did we really need to be restricted to three pieces a day AND have the pieces go to our inventory AND only have certain games show up on certain days? I think only one or two of those alterations would have been sufficient.
Theoretically, yes, one could trade their extra pieces for ones they don't have, but that's yet another hassle. Especially if you have one that everyone ELSE is trying to get rid of. So it's easier to just sell them. But then if you use the money from those sales to buy the pieces you want, then sometimes the map winds up costing more...
whoops, I said I wasn't going to rant. Sorry.
Can you tell how much I hate the system?