why is a round trip fare between London and Paris cheaper than a one way fare on Eurostar? round trip fare is $120 and the one way fare is $177.
Is it possible to buy a round trip ticket and just use the outbound only?

Their logic is that people buying a round trip are mostly leisure travellers who will go elsewhere if Paris is too expensive, whereas someone trying to buy a one-way ticket must need to get to Paris so is willing to pay whatever Eurostar charges.
Your solution is sensible: buy a return ticket and only use the outward part. Lots of people do it. When choosing your fictitious journey back, try to pick a slack time of day in the middle of the week when fares will be lowest.

there is a byzantine pricing structure and i think you may be mistaken that on the same train you pay $177 one way and $120 round trip - yes you can use the round trip to go one way - nothing to prevent that. Folks also do that with the $102 day return tickets, which albeit are very hard to book.

Check www.eurostar.com for prices in British pounds which can often be cheaper and in the US check with raileurope, which is where you got these prices from it seems - and the raileurope web site often does give such perplexing info - this is why i always advise talking to a real person - an expert who would do a manual search as the raileurope.com site often gives such misleading info IME. I always advise calling BETS (800-441-2387) for the best possible service for fares thur RailEurope and be sure to check www.eurostar.com for fares in pounds or euros if coming from Paris - your ticket will be sent to you and many have done this easily. But check both sources, here and there and take the best as they often don't corresponds in having exact or similar fares.

"bmwf1" is not mistaken. It is often much cheaper to buy a return Eurostar ticket than to buy a one-way ticket, travelling outwards on the same train.

I did go on www.eurostar.com and got these fares. Why would anyone buy a one way ticket if that is the case?

<<< Why would anyone buy a one way ticket if that is the case? >>>

For Americans that isn't the case - the cheapest 1 way fare for them is $67 with the cheapest return at $72

For other folk though the cheapest one way fares allow you to change the train you travel on or even get a refund, the cheapest return tickets are "use it or lose it" and you can't change anything

When you have the same TYPE of ticket the return fare is generally 50% more than the single fare - just like the airlines

alan has explained it to me and i should have thought of that - ticket conditions can vary and flexible ones can cost much more.

some folks want flexibility and will pay more - a return flexible ticket would be much more.

that's what makes the senior fare for 60+ so good - starts at $94 in US and can be changed once in Europe up until the time of the train - ditto for passholder fares which start at $77 or so.

But the caveat is changeable subject to availability in that fare category and that is not always the case that it in reality can be changed on short notice. But better always than a non-changeable ducat.

No difference from many US domestic air tickets.

For the sme reason that a one-way ticket on an airplane is more expensive than a RT.

Yes, you can travel one way and not use the return ticket.