Certainly start looking someone close by, your colleagues, classmates, friends and families. This makes the whole process easier and you normally get to sell it with a more reasonable price.
To know an estimate of how much it might be sold for, try eBay search for a similar IBM, then click the "Completed Listing" at the left column to refine your search. This will give you a rough idea how much it has been sold in the past.
Yep, I am with Eric Lim, try selling it to your friends and family. If you think the price is a bit on the low side, get some opinions on what it is roughly worth and then negotiate with your friend, to see if you can come to sort of arrangement.
If that fails and you want to try and get a better price for it, the next best thing would be local paper classified ads. This is much more convenient for both you and the buyer then on eBay as it means you will be able to take it to them/they pick it up instead of relying on a courier company.
Other then this, eBay is your best place to sell it. However, you may not get full price for it here, it all depends on who is looking for a similar at the time and how much they are prepared to pay for it, plus many people are weary about buying expensive things via eBay.
Whatever you choose and whatever happens, good luck with the sale
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