Some one is selling me a Yamaha YDP-121 Digital Piano for $450.00.
Any advise?

The YDP-121 is ancient. You might be able to find a better keyboard (but not a console), brand new, for not much more money.

People here might be better able to help if you'd call out some of your needs.
- Advanced player or beginner?
- Console or keyboard?
- How much would you willingly spend?
- How important to you is the keyboard feel?

Before buying a used piano, always look at what the same money would buy in a brand new piano. The $500 casino has much better specs then the used YDP121.

If you could get the YDP for $200 or even up to $250 then I'd say they is nothing on the market that is better for that price. And then yo'd use you computer to generate the sound.

Always offer half price or less for a used digital (or acoustic)
piano. Any piano is hard to sell because the number of
players in the population is small. It has no warranty and
you can buy good new digitals with full warranty for about
the same price. There's nowhere the seller can unload it
fast, since a piano dealer would never buy it and a pawn shop
will give him almost nothing for it. No one is going to be lining
up to buy this, and you're probably the only person who is
interested in it. Offer half price or less for it and the seller
should grab for that because it's probably the only offer he
will get.