Sell Or Build?

I was given a round pen; not a metal one that can be used in a million sorts of ways, but a wooden post and rail one. It had only been in the ground for a short time, maybe a year, so it's in good condition still. The wood is treated so no rot at all.
There are 11 posts about 8 inches round and 8 feet tall and 28 rails. The posts have notches in them for the rails to go into and built it is about 40 feet across.
Now.... I have the 50 X 60 covered arena so I don't need a round pen besides that, once the stalls are built we will be able to lunge in the arena with plenty of room. So the posts and rails have been sitting in a pile for 4 months waiting to be used.
The wood posts and design of them doesn't match any of our other fences, I'd really like to have just field fence and white rail fences, not a third kind. So I put an ad on Craigslist to sell this round pen for $300 and so far I have 3-4 people interested in it. In fact, someone wants to come get it this evening.
But now I'm thinking maybe we could use it to build an 8x8 wash rack for the horses. We have the location for it, between the chicken house and the side of the old stalls, where the new barn door will be. There is an empty space with jumbled up roots and dirt clods, we could use the tractor to dig it out, auger out 4 post holes, then put in the corner posts and the rails, finishing it off with a load of gravel. To make it a bit nicer we could add 2x10s around the bottom to contain the gravel and stall mats over the top. I've got the water heater...we could also put over-the-rail shelves and the over head hose holder, if we wanted.
I think we could get this built in one single day, it's not overly complicated and it would look nicer than plain old 4x4 posts and 2x4s. So now I don't want to sell it. So what in the hell would I do with the rest of the posts and rails? They are nicer quality materials than Lowes would sell me, for a lot of money.
I'd really like to bath my horses in a nice, safe area where they can get stupid but not get hurt, where they can see the other horses without getting to them and messing around.
Let's see if LAN is on board with this.