Buyer beware! There is much fake Indonesia money on ebay.
In general the revolutionary period notes (dated 1945-1949) are most likely to be fake. The fakes are usually old fakes, and have some collectors' value of their own, but they were never real money. The problem with money from this period is that the government was at war, and so they were not able to outsource banknote printing to secure printers such as Thomas De La Rue or the American Banknote Company. Hence these notes lack watermarks, sophisticated printing, or fancy paper. Unfortunately it is impossible to prove for certain which notes of the 1946 (the 1945 series was not issued until October 1946 - these were the first Republik Indonesia banknotes) -1949 period are genuine, because equipment varied, paper was being captured in war, chemicals and supplies ran out, etc. You can however show for certain that some are fake, mostly by means of the serial number.
In addition, generally higher denomination banknotes are more likely to be fake.
There are also some modern fakes of Indonesian money created to deceive collectors.
The absolute number 1 fake on sale on ebay is the 1948 400 Rupiah note. I have looked at about 20 auctions for this note. All are fakes and have a limited value, $10 in UNC at the absolute most. The REAL notes are worth the price listed in the catalogue.
The fake notes can be identified by the four letters that are part of the serial number.
If the note on sale has any of the following sets of four letters, it is FAKE, 100% for sure, do not buy it, unless it is advertised as a fake, because other buyers will pay over the odds for it:
The following letters are MOSTLY fakes:
Except that notes with serial numbers 08xxxx BK NI MIGHT be genuine (if it does not begin 08 but has BK NI letters it IS fake), and notes 01xxxx MD FC MIGHT be genuine (again, if it doesn't begin with 01 but has MD FC code, it is probably fake). The 08 BK NI notes are quite likely to be fake too. So it is safer to avoid BK NI altogether, since it's a very common series of fakes.
Another commonly faked note is the 1945 10 Rupiah note. Buying a fake of this is not going to be as bad as the 400 rupiah, because the value of a real one is only around $10. Even so, slightly more than half the notes on ebay labelled as 1945 10 Rupiah are actually fake. These fakes can be identified by measurement, or before sale by looking at the serial number and the two letters of the suffix.
Real notes will match the first number of the serial to the first letter of the suffix.
So for REAL (not fake), then if the serial starts 0, the suffix starts X.
If the serial starts 1, the suffix starts either D, E, or F.
If the serial starts 2, the suffix starts either K, L, or M.
If the serial starts 3, the suffix starts either R, S, or T.
If the serial starts 4, the suffix starts Z.
If the serial starts 5, the suffix starts either A, B, or C.
If the serial starts 6, the suffix starts either G, H, or I.
If the serial starts 7, the suffix starts either N, O, or P.
If the serial starts 8, the suffix starts either U, V, or W.
If the serial starts 9, the suffix starts Y.
Anything not matching this pattern is counterfeit.
Another annoying fake is the 1947 5 rupiah. These all have the serial number 1xxxx EA. They are extremely common, and always seem to be sold in uncirculated condition. If you look at the picture on the auction you will notice there is no underprint. Genuine notes have a large number five and other patterns printed in the middle of the note. These fakes do not.
In the slightly more sophisticated category of fakes, the 1952 series has fake 100 and 1000rp notes.
The fakes can be quite identified by examining the reverse of the note. In the fake, there will be eight wavy yellow lines running all the way from the top to bottom of the note (including the border area). These are about 5mm thick. If these are present, the note is fake. These lines are an attempt to simulate the watermark (which is indeed 8 wavy lines), but the difference is that a genuine watermark should be visible on holding up the paper to the light, not overprinted as the fakes are. Also the serial numbers: 'WA' for the 1000 rupiah, and 'ZG' for the 100 rupiah, are also alarm bells for fakes - look closely at the pictures.
If the seller does not show both sides of the note, you should not buy anything from them anyway, because you cannot judge anything without seeing both sides.
If the picture of the note is too small, do not buy it. Ask for a bigger picture and encourage the seller to use auctiva large pictures.