I don't see it being worth less $5k+ if it's really 10x4x4. I also don't see it being a smart move if it's thin acrylic. If you're hoping to get it for 2 or 3x what you paid for your 450, you're nowhere near the ballpark of their $5600 and I don't see how you meet in the middle. Also, even with the stand being "free", you still kinda haveta figure it in.
New Tenecor 10x4x4 is $10k without stand, even $5600 might be a comparative steal if the acrylic was thick enough. If you're in the market for a 10x4x4 the price is probably great but if you're in the market for a cheap big tank it's probably not gonna be cheap enough.
Drilling and adding a coast-to-coast overflow at the top should be relatively simple and wouldn't require thick acrylic so would be reasonably cheap.